Conformational Stability and Intramolecular Hydrogen Bonding in 1, 2

Subscriber access provided by SETON HALL UNIV

Article

Conformational Stability and Intramolecular Hydrogen Bonding in 1,2-Ethanediol and 1,4-Butanediol Prasanta Das, Puspendu Kumar Das, and Elangannan Arunan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp512686s • Publication Date (Web): 03 Apr 2015 Downloaded from http://pubs.acs.org on April 5, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Conformational Stability and Intramolecular Hydrogen Bonding in 1,2-Ethanediol and 1,4Butanediol Prasanta Das*, Puspendu K. Das and E. Arunan Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

Received: December 19, 2014; Revised Manucsript received: March 15, 2015 To whom correspondence should be addressed. E-mail: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The gas phase infrared spectra of 1,2-ED and 1,4-BD have been recorded at three different temperatures using a multi-pass gas cell of 6 meter optical path-length. DFT calculation has also been carried out using 6−311++G** and aug-cc-pVDZ basis sets in order to look for the existence of intramolecular hydrogen bonding in them from the red-shift and infrared absorption intensity enhancement of the bonded O−H band compared to the free O−H band. Equilibrium population analysis with 10 conformers of 1,2-ED and 1,4-BD at experimental temperatures were carried out for the reconstruction of the observed vibrational spectra at that temperature using standard statistical relationships. The most abundant conformer at experimental temperatures was identified. In 1,2-ED a red-shift of 45 cm−1 in the intramolecularly interacting O−H stretching vibrational band position and no significant intensity enhancement compared to the free O−H have been observed. On the other hand, in one of the hydrogen bonded conformers of 1,4-BD, a 124 cm−1 red-shift in the O−H stretching frequency and a 8.5 times intensity enhancement for the ‘bonded’ O−H compared to the ‘free’ O−H is seen. Based on this comparative study, we have concluded that strong intramolecular hydrogen bonding exists in 1,4-BD. But there appears to be weak intramolecular hydrogen bonding in 1,2-ED at temperatures of 303, 313 and 323 K in the gas phase. We have found that most stable hydrogen bonded conformers of 1,4-BD are less populated than some of the non-hydrogen bonded conformers. Even for the 1,4-BD, the relative population of the g′GG′Gt conformer, which has strong intramolecular hydrogen bond, is less than what is predicted. Perhaps the intramolecular hydrogen bond plays a less significant role in the relative stability of the various conformers than what has been predicted from calculations and prevails in the literature.


Page 2 of 50

Page 3 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


KEY WORDS: Diols, 1,2-ethanediol(1,2-EG), 1,4-butanediol (1,4-BD), hydrogen bonding, gas phase FT-IR spectroscopy, DFT calculation 1. INTRODUCTION The molecular structure, chemical reactivity, and spectroscopic features are frequently rationalized in terms of the conformation of molecules. Intramolecular hydrogen bonding can be a critical element in deciding the shape of a molecule. This importance is evident in the structure of molecules containing multiple functionalities, such as diols, that can engage in hydrogen bonding. Diols exist in a mixture of conformers with several possibilities of intramolecular and intermolecular interactions between the two hydroxyl groups depending on the temperature and the physical state. The extent of intermolecular interaction is dependent on the concentration of the compound, while intramolecular interaction is concentration independent. Intramolecular hydrogen bonding (IHB) is particularly sensitive to changes in the molecular geometry. Ethylene glycol or 1,2-ethanediol (1,2-ED) is one of the simplest molecules with two vicinal hydroxyl groups which serves as a prototype for understanding the influence of hydrogen bonding on the conformation of biological molecules and for finding how IHB stabilizes a particular conformer. Whether or not 1,2-ED form an intramolecular hydrogen bond has a large significance since this molecule has been used as a part of the training set for parametrizing molecular mechanics programs.1 It is used in pharmacologically active materials2 and in polymer synthesis.3 In 1,2ED, the IHB conformation will form a five-member quasi-ring. The hydrogen bond angle in such a five member ring is far from an optimal 180° and thus the hydrogen bond is expected to be weak. On the other hand, butanediol isomers are compounds with numerous applications such as protein-stabilizing agents. Butane-1,4-diol or 1,4-butanediol (1,4-BD) is an interesting member



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of the diol family with many possibilities of intramolecular interaction between the two hydroxyl groups. In the past, structure of diols including 1,2-ED and 1,4-BD in the gas, liquid and solid phases have been studied extensively by different experimental methods such as X-ray, neutron4,5 and electron diffractions,6,7 and microwave,8,9,10 NMR,11,12,13 and FT-IR spectroscopy.14,15 The electron diffraction studies on 1,2-ED in the gas phase indicated that the gauche conformer is considerably more stable than the trans because of the presence of the internal hydrogen bonding in the gauche conformer.7 This has been corroborated by neutron diffraction studies in the liquid phase.4 X-ray studies in solid confirmed the preferred gauche conformers but did not show evidence for hydrogen bond.5 The NMR investigation of 1,2-ED,13 however, led to the conclusion that intramolecular hydrogen bonding between the hydroxyl groups is unlikely to be a significant factor in determining the preference for gauche conformation except in fairly nonpolar solvents. For the past 20 years, extensive quantum-chemical calculations have been carried out for the conformational analysis and characterization of the hydrogen bond between two vicinal hydroxyl groups in ED.16,17,18,19,20,21,22,23,24,25,26,27,28 Electron density topological analysis has not shown a bond critical point (BCP) and atomic bond path corresponding to IHB in 1,2-ED.5,29,30 However, Klein argues that the absence of a bond critical point does not imply that the absence of a bond although the presences need not necessarily indicate the presence of a bond.23 The quantum diffusion Monte Carlo (QDMC) simulations in the past31 and X-ray crystallographic studies on crystalline 1,2-ED suggests that the intramolecular hydrogen bond (IHB) does not exist in this molecule.5 Cheng et al. did the conformational analysis with 1,2-ED, 1,3-PD (propane diol) and 1,4-BD using B3LYP/6-31+G(d,p) and QCISD/6-311++G(3df,3pd) computational method to 4 ACS Paragon Plus Environment

Page 4 of 50

Page 5 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


find the origin of disappearing of hydrogen bonded O−H peak in overtone spectra of diols.28 They have pointed out that higher energy conformers are needed to reproduce the experimental overtone spectra of diols. Based on Non-Covalent Interactions (NCI) analysis on 1,2-ED, 1,3-PD and 1,4-BD, Kjaergaard and coworkers argued that the BCP criterion of AIM theory is too stringent and the absence of a BCP should not necessary be considered evidence for absence of hydrogen bond.32 X-ray, electron diffraction and NMR spectroscopy give structural information of the bulk system in the condensed phase, whereas IR spectroscopy is an ideally suitable method to get structural information of a monomer in the gas phase. This method has been used for the identification and characterization of hydrogen bonding. Generally, hydrogen bonding manifests in a red shift of the O−H stretching frequency and broadening of the width. For the hydroxyl group, the increase in infrared absorption band intensity and a red-shift of the O−H stretching frequency are spectroscopic signatures of hydrogen bond formation. The magnitude of both the intensity and frequency shift increase with increasing hydrogen bond strength.33 Combination of FT-IR spectroscopy and calculation on 1,2-ED and 1,4-BD in the gas phase is very limited. Only a handful of reports on IR absorption spectroscopic studies on gaseous 1,2-ED and 1,4-BD are available in the literature.14,15 Conformational isomers of 1,2-ED have been investigated by IR spectroscopy in the gas (up to 125 oC), liquid and solid phases in 1967 by Buckle et al.14 The gas phase result of Buckle et al. shows a 33 cm−1 red-shift which was used in support of the evidence for the presence of the gauche conformer in the gas phase. The infrared spectroscopy of 1,2-, 1,3-, 2,3-, and 1,4-BD have been studied in the gas and liquid phases at ± 2 cm−1 resolution by Fishman et al.15 They found a large red-shift of 110 cm−1 in 1,4-BD. Their results also showed that the enthalpies of IHB formation in these isomers depend strongly on their conformation. In 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2003, Jesus et al. did conformational analysis of BD isomers by DFT calculations as well as IR spectroscopy in an inert (CCl4) solvent and in pure liquid.34 The conformations of 1,4-BD have been investigated using a combination of MP2 calculation, natural bond orbital (NBO) and AIM analysis, and matrix-isolation infrared spectroscopy by the same group in 2008.35 They found that the two intramolecular hydrogen bonded gauche conformers contribute 46% in the equilibrium conformation at room temperature. However, their matrix isolated spectrum of 1,4BD did not agree with the simulated spectrum particularly in the bound O−H stretching region. A large intensity discrepancy between the observed and the simulated spectra is also seen, although they did not discuss the intensities of the observed bands in the paper. In 2009, Ma et al. carried out IR spectroscopic study and DFT calculation on 1,2-ED, 1,2-PD, 2,3-BD, and 1,2-BD in solution.36 They found that the red shift was of the order of 40 cm−1 in the vicinal diols and attributed it to IHB. Vapor phase O−H stretching overtone spectra of 1,2-ED, 1,3-PD, and 1,4BD have also been reported in the past and the existence of small red-shift in the 1,2-ED spectrum has led to the conclusion that IHB is weak in 1,2-ED molecule in the gas phase.27,37 An interesting question which arises from all these reports is that whether IHB is found between vicinal hydroxyl groups in 1,2-ED in the gas phase or not. Chopra et al.5 has pointed out that observation of a small red-shift in the vibrational frequency is not sufficient to conclude the presence of IHB in the absence of NMR evidence or of a BCP from a theoretical electron density study. However, based on AIM calculations on 1,2-ED, Arunan et al., recently, suggest that IHB can appear during a vibration which is dominated by the O…O stretching.38 Most of these previous studies are silent about intensity changes which are expected to be there due to IHB formation. We have chosen two specific diol systems namely 1,2-ED and 1,4BD to investigate if intensities play an important role in dictating the most prevalent conformer 6 ACS Paragon Plus Environment

Page 6 of 50

Page 7 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in the ground state of the diols. Our goal in this work is (1) to record the IR spectra of 1,2-ED and 1,4-BD at the lowest possible concentrations in the gas phase where intermolecular interaction is minimum/absent, (2) to calculate the equilibrium population of each conformers of 1,2-ED and 1,4-BD at the experimental temperatures, (3) to identify the conformers of 1,2-ED and 1,4-BD with the help of the simulated population weighted spectrum and the observed spectrum at a particular temperature, and (4) to answer under which conditions does IHB exist in the two diols, 1,2-ED and 1,4-BD. 2. EXPERIMENTAL SECTION The sample 1,2-ethanediol (HOCH2CH2OH, LR grade, Ranbaxy) and 1,4-butanediol (HOCH2CH2CH2CH2OH, 98%, Merck) were used following several cycles of freeze-pump-thaw to remove dissolved gases. Details of the experimental set-up have been described elsewhere.39,40 In brief, the gas phase mid-IR spectra were collected using a Vertex-70 FT-IR spectrometer (Bruker Optics, Germany) coupled with a 6 meter heated multi-pass long-path gas cell (Model 7.2-V, REFLEX Analytical Corporation). The cell body is a cylinder of borosilicate glass of length 20 cm with an inside diameter of 6 cm. It contains three gold coated mirrors. The FT-IR spectrometer was equipped with liquid-N2 cooled HgCdTe (LN-MCT) detector/KBr beamsplitter combination. The sample was placed in a ~2 ml bulb attached to the long-path gas cell and vacuum line through on-off valves. First, the sample was degassed by pumping and subjecting it to several freeze-pump-thaw cycles to remove dissolved gases. The vacuum line was then closed and the sample vapor was allowed to equilibrate with the cell at 303 and 313 K for 1,2-ED and 1,4-BD, respectively. The gas cell and sample bulb containing diols were heated with the help of heating tape and the temperature of the cell was maintained with the help of temperature controller fitted with a feedback heat sensor. The sample vapor was subsequently 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

diluted with 100 mmHg Ar (buffer gas) in order to minimize the intermolecular interaction. All spectra were recorded with spectral resolution of 0.5 cm−1 and averaged over 2048 scans. The spectra were recorded at three different temperatures with increments of 10 °C in each stage from 303 K to check for the existence of intermolecular interaction. 3. COMPUTATIONAL METHODS Ethane-1,2-diol is a typical rotor molecule that can exist in one of the 33 = 27 conformations. Some of the structures are degenerate due to symmetry, and the number of unique conformations is 10, as shown in Figure 1.27 The conformers are defined according to the following rule: assuming three possible minima per torsion, i.e., trans (180o ± 30o), +gauche (60o ± 30o), and – gauche (−60o ± 30o) abbreviated by t or T, g or G and g′ or G′. Capital letters refer to the backbone structure while the small letters refer to the O−H orientation and superscript ‘prime’ indicates the sense of internal rotation corresponding to the negative dihedral angle. In the case of 1,4-BD, there are five intramolecular rotational degrees of freedom with three of them characterizing the backbone structure (OCCC, CCCC and CCCO) and the remaining two are related to the orientation of the two OH groups (HOCC). The existence of three-fold axis around each of the five dihedral angles can result in a total 35 = 243 conformations that complicate the structural studies of 1,4-BD. However, symmetry considerations reduce this number to 70 unique conformations.35 Out of these, we have taken the most stable conformer from each backbone family which adds to a total of 10 conformers. We expect percentage population of higher energy (∆E0 >10 kJ mol−1) conformers in the same family to be insignificant35 and, thus, we have chosen 10 conformers to simplify the analysis.


Page 8 of 50

Page 9 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The optimized geometries, both harmonic and anharmonic frequencies and intensities, and thermodynamic parameters on 10 conformers of 1,2-ED and 1,4-BD were obtained with the DFT (B3LYP) method using 6-311++G** basis. We have also done the calculations at the B3LYP/aug-cc-pVDZ level of theory since we were unable to get satisfactory results with B3LYP/6-311++G**. We have carried out frequency calculation on conformers of 1,2-ED and 1,4-BD to ensure that we were dealing with conformations at the true potential energy minima and not at the transition states or saddle points. In case of the g′Gg′ conformer, however, one imaginary frequency at 75.1 cm−1 has been observed when calculations carried out at the B3LYP/6-311++G** level of theory indicating the presence of a first-order transition point and not a true minimum. This has been pointed by Klein earlier.29 Graphical examination of the DFT calculated vibrational frequencies using Gauss View 5.0, showed that the single imaginary frequency corresponded to a wagging mode for the pendant OH groups. Moreover, it has been found that geometry optimization of the conformer tGt led to the conformer g′Gg′. Therefore, the conformer tGt does not correspond to a stationary point at this level of theory and, thus, we will not consider it further for the population analysis at the B3LYP/6-311++G** level of theory. All computations were carried out using the Gaussian 09 set of programs.41 We have performed a statistical thermodynamic population analysis at experimental temperatures with the chosen 10 conformers of 1,2-ED and 1,4-BD. The purpose of the theoretical population analysis is to predict which conformers contribute to the vibrational spectra. Standard statistical mechanical relations were used to calculate the free energies and, thus, relative populations of conformers of 1,2-ED and 1,4-BD in a manner similar to that performed by Howard and Jesus.27,35 In brief, the electronic energy values obtained after geometry optimization were corrected with the zero-point vibrational energy giving total energy



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 50

at 0 K (E0 = Eelec + ZPE). The translational, rotational and vibrational thermal energies were added to this value in order to obtain the enthalpy (H = E + RT, where E = E0 + Evib + Erot + Etrans) at three different experimental temperatures for 1,2-ED and 1,4-BD. From this quantity and calculated entropy and the Gibbs energy (G = H−TS) are determined. The relative weight of each conformer in the gas phase was obtained from the Boltzmann distribution based on the Gibbs energy, see section 4.2 for details. 4. RESULTS AND DISCUSSION 4.1. Gas phase infrared spectra of 1,2-ED and 1,4-BD. The gas phase infrared spectra of diols has been reported at 398 K for 1,2-ED14 and at 573 K for 1,4-BD.15 We have measured gas phase mid-infrared absorption spectra of 1,2-ED and 1,4-BD at three different temperatures up to 323 and 333 K, respectively, and presented in Figure S1(supporting information). Frequencies, FWHM and band areas of the O−H stretching vibrations for 1,2-ED and 1,4-BD as a function of temperature are listed in Table 1. We note that in the gas phase both the in/bound O−H and the out/free O−H stretching frequencies can be seen at experimental temperatures. The ‘in’ and ‘out’ abbreviation used for two OH groups in 1,2-ED whereas in 1,4-BD it is ‘bound’ and ‘free’, respectively, as described in Figure 3. From Figure S1(supporting information), we can see that the frequencies of both of these bands are independent of temperature since the red-shift (∆ν in cm−1) in O−H stretching frequency is constant as the temperature is increased. This implies that there is no intermolecular interaction. If intermolecular interaction is present, the O−Hin/bound stretching frequency as well as intensity are expected to change with temperature. However, intensities of the observed bands are found to increase as a function of temperature since the gas phase concentration of the sample becomes more at higher temperatures. Furthermore, it has been found that the ratio of integrated band area (R) of O−Hin/bound to the O−Hout/free at higher 10 ACS Paragon Plus Environment

Page 11 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperatures of 323 and 333 K for 1,2-ED and 1,4-BD, respectively, is slightly smaller than those at lower temperatures of 303 and 313 K. This clearly implies that the conformational equilibrium between intramolecularly interacting and noninteracting forms is shifted towards the noninteracting form by a small extent at higher temperatures. This has been evidenced from the gas phase equilibrium population analysis also, see discussion in the next section 4.2. Out of these three sets of spectra one set of spectrum measured at 303 K for 1,2-ED and 313 K for 1,4BD was chosen for the identification of conformers in the gas phase by comparing with the simulated spectrum. 4.2. Population analysis and identification of conformer. The comparison between the experimental and simulated spectra allows us to verify the presence of the most stable conformers in the gas phase. In order to interpret the experimental spectra, we considered 9 or 10 conformers out of 27 for 1,2-ED; depending on method of calculation employed and 10 conformers out of 243 conformers in 1,4-BD for conformational analysis. The relative electronic energies (corrected for zero-point vibrational energy, ZPVE), ∆E0 and relative free energies, ∆G, of selected conformers of 1,2-ED and 1,4-BD calculated with the B3LYP/6-311++G** and the B3LYP/aug-cc-pVDZ methods are presented in Tables S1 and S2 (supporting information), respectively. Each conformer has a contribution to its free energy of –RTlnω, where ω is the structural degeneracy of conformers listed in Figures 1 and 2 for 1,2-ED and 1,4-BD, respectively. The fractional gas-phase equilibrium population,17,27,35 P(M) of a conformer M is calculated according to the Boltzmann distribution P( M ) =

exp( − ∆G M / RT ) ∑ exp(−∆Gi / RT )

••••••••••••

(1),

i



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 50

where i spans all the conformers of 1,2-ED and 1,4-BD, respectively. Gibbs free energies were taken from Tables S1 and S2 of supporting information. The calculated P(M) at experimental temperatures were multiplied by 100 in to order to get A(%). The calculated % population of chosen conformers of 1,2-ED and 1,4-BD are listed in Tables S1 and S2 (supporting information), respectively. We assumed that population of each conformer corresponds to its weight at the experimental temperature. For simulation of spectra at experimental temperature, we have used anharmonic frequencies and intensities obtained with B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ. The anharmonic intensity of some bands in the C−H stretching region and at low frequency vibrational mode region were over estimated by both methods; in those cases harmonic intensities were used for the simulation of IR spectra. In case of the g′Gg′ conformer in 1,2-ED, we were unable to obtain anharmonic frequencies and intensities at the B3LYP/6-311++G** level of theory. In this case, the harmonic frequencies of the calculated spectra were scaled by three scaling factors: 0.9578 for O−H stretching region, 0.9641 for methyl C−H stretching region, and 0.9824 for below 1600 cm−1 region. The scaled factors for each region were calculated by taking the ratio of νobs/νcal with respect to the most intense band. Calculated spectra were simulated using Gaussian functions centered at the calculated scaled frequencies and with band width at half height equal to 35 cm−1. The calculated infrared intensities of the spectra of each conformer were weighted by its population as described in Tables S1 and S2 (supporting information). The experimental and B3LYP/aug-cc-pVDZ simulated spectra of 1,2-ED and 1,4-BD are presented in Figures 4 and 5, respectively. The stimulated spectrum (at 303 K) of 1,2-ED, a mixture of conformers: tGg′, gGg′, g′Gg′,tTt, tTg, gTg′, gTg, gGg, tGt, and tGg, with a 12 ACS Paragon Plus Environment

Page 13 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contribution of 55.5, 22.6, 13.4, 0.5, 2.8, 1.3, 0.4, 1.0, 1.0, and 1.5%, respectively, is shown in Figure 4(B). From Table S1, it is clear that conformer tGg′ in 1,2-ED is the most populated one followed by gGg′ and g′Gg′ conformers at 303 K. Out of these, the conformer g′Gg′ is unlikely to have IHB. The experimental spectra of 1,2-ED match well with the simulated spectra obtained with the B3LYP/aug-cc-pVDZ method. It implies that the calculated % populations at a particular temperature agree well with the experimental populations. From Figure S2 of supporting information, we can see that the experimental spectra do not match well with the B3LYP/6-311++G** simulated spectra, particularly in the O−H stretching region. This is perhaps due to under estimation of O−Hout stretching frequency. The red-shift due to H-bonding calculated at the B3LYP/6-311++G** level of theory is found to be small (23 cm−1) compared to the observed red-shift (45 cm−1). As a result both O−Hin and O−Hout stretching frequencies appear under same band in the B3LYP/6-311++G** simulated spectra. Thus, at this level of theory, we were unable to judge whether the estimated percent population of conformers in 1,2ED reproduces the experimental population or not. The simulated spectrum (at 313 K) of 1,4-BD derived with the B3LYP/6-311++G** method from a mixture of conformers: g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt with weightages of 25.1, 7.6, 18.5, 18.4, 14.8, 3.9, 1.8, 5.6, 3.0, and 1.3%, respectively, is shown in Figure S3 (B′) (supporting information). In the simulated spectrum the H-bonded O−H peak intensity is much larger than what is seen from the experimental spectrum. This discrepancy between simulated and observed spectra at 313 K in 1,4-BD is consistent with the matrix-isolation infrared spectroscopic study of 1,4 BD and MP2 level calculations reported by Jesus et al.35 This indicates that the calculated % population of conformers is different from the % population under experimental conditions. Therefore, we 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 50

employed another method B3LYP/aug-cc-pVDZ to find the experimental % population of various conformers in the gas phase and the same has been reported in Table S2 (supporting information). The simulated spectrum obtained with the B3LYP/aug-cc-pVDZ calculated anharmonic frequencies, intensities and % populations is displayed in Figure 5(B′). The populations estimated with the B3LYP/aug-cc-pVDZ method match the experimental spectrum better and are found to be 12.8, 4.4, 21.8, 26.8, 18.8, 3.3, 1.9, 5.1, 3.9, and 1.2% for g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt conformers, respectively. However, in case of second hydrogen bonded gG′G′Gt conformer, we see that the population is lower than 4.4%, as calculated with the B3LYP/aug-cc-pVDZ method. This has also been evidenced from O−Hbound/O−Hfree band area ratios of 0.7 which is smaller than the expected value of 3.2, as listed in Table 3. The lower population of hydrogen bonded conformers in 1,4-BD implies that in spite of IHB, the two hydrogen bonded conformers are less stable than some of the non-hydrogen bonded conformers. Point to be noted is that the observed band area listed in Table 1 for the out/free O−H group in 1,2-ED and 1,4-BD has a contribution from all the conformers chosen for the spectral analysis. Therefore, observed band area for the hydrogen bonded conformers is obtained by weighted percent population of the total band area at 3682.8 cm−1 in 1,2-ED and 3672.5 cm−1 in 1,4-BD and presented in Tables (2 – 3). The calculated anharmonic vibrational wavenumbers and their anharmonic intensities of the O−H stretching band of 1,2-ED and 1,4-BD are listed in Tables 2 and 3, respectively. The experimental and simulated spectrum in the O−H stretching region of 1,2-ED and 1,4-BD are shown in Figure 6. In the O−H stretching region, the gas phase IR spectra of the 1,2-ED exhibit two overlap bands, whereas 1,4-BD exhibits three distinct bands, as found in Figure S1(a−b) 14 ACS Paragon Plus Environment

Page 15 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(supporting information). The higher frequency band appears at 3682.8 and 3672.5 cm−1 in 1,2ED and 1,4-BD, respectively. This band corresponds to the stretching vibration of the out/free O−H group and has a contribution of all the conformers which are used for the simulation of the calculated spectra. The second O−H stretching band observed at 3637.5 cm−1 in 1,2-ED is assigned to the O−Hin stretching vibration. Only two conformers tGg′ and gGg′ of 1,2-ED are contributing to this band. In 1,4-BD two bands are observed at 3548.4 and 3606.5 cm−1. The appearance of two hydrogen bonded O−H stretching bands provide evidence for the existence of two differently hydrogen bonded conformers, namely g′GG′Gt and gG′G′Gt, respectively. From Table 3, we can see that IHB red-shift in strong hydrogen bonded conformer g′GG′Gt of 1,4-BD is found to be (171 − 177) cm−1 at the B3LYP/6-311++G** and the B3LYP/aug-cc-pVDZ level of theories, which is close to the red-shift (166 cm−1) predicted with the MP2/6-311++G(d,p) method.35 However, it deviates by ~50 cm−1 with respect to the observed red-shift. A more sophisticated method of calculation is required to describe properly the potential-energy surface corresponding to the O−H vibrational mode. A low intensity band observed at 1754.7 and 1820.4 cm−1 for 1,2-ED and 1,4-BD, respectively, does not correspond to any calculated frequency. Therefore, this band is tentatively assigned as a non-fundamental band. However, it is not clear which of the conformers contribute to this band. The experimental spectra of 1,2-ED match well with the simulated spectra obtained at the B3LYP/aug-cc-pVDZ level of theory as discussed earlier, which implies that the calculated % populations at this level of theory agree well with experiment. From Figure 4 and Table S1 in supporting information, it is clear that, in case of 1,2-ED most stable conformers, tGg′, gGg′, and g′Gg′, that is, the gauche conformers are present in the gas phase. On the other hand, in 1,4-BD



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 50

comparison of the experimental spectrum with the B3LYP/6-311++G** simulated spectrum reveals that although frequencies match well, the relative intensities of the bound O−H band observed at 3548.4 and 3606.5 cm−1 do not agree with the simulated spectral intensities, as seen in Figure S3. The discrepancy between observed and calculated relative intensities in 1,4-BD suggests that intramolecular hydrogen bonded lowest energy conformers, g′GG′Gt and gG′G′Gt do not contribute 25.1 and 7.6%, respectively, to the population in the equilibrium mixture, and rather it is far less. Indeed, the values are found to be 12.8 and 4.4%, respectively with the B3LYP/aug-cc-pVDZ level of theory. Even if we consider B3LYP/6-311++G** calculated percentage population as upper limit, the total contribution of the IHB conformers to the equilibrium composition calculated by us is ca 32% which is much less than the 46% contribution calculated by Jesus et al.35

4.3. Estimation of hydrogen bond energy. The peak frequency shift, ∆ν = (νout/free−νin/bound) is commonly used to evaluate the strength of the hydrogen bond and a correlation between this and the enthalpy of the hydrogen bond formation has been reported by several groups. The HB formation enthalpy can be estimated using the empirical relation proposed by Iogansen,42 ∆H = −0.3312( ∆ν − 40)1 / 2

••••••••••••

(2)

in which ∆H is expressed in kcal mol-1 and ∆ν in cm−1. Equation (2) is valid only when ∆ν≥ 40 cm−1. This relation also implies that the minimum for hydrogen bond induced red shift is at 40 cm−1. The red-shift and estimated HB formation enthalpy of the 1,2-ED and 1,4-BD are listed in Tables 2 and 3, respectively. The red-shift in O−H stretching vibration of 1,2-ED is found to be 44.2 ± 1.6 cm−1 which is slightly higher than the previously reported shift 33.0 cm−1 in the gas


Page 17 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


phase,14 35.6 cm−1 in CCl4 solution,36 and 39.1 cm−1 in solid Ar,43 whereas calculated red-shifts are found to be 47 and 32 cm−1 for tGg′ and gGg′ conformers, respectively. It complicates the observed IR spectra in the O−H stretching region. From Table 2, we can see that in calculation, some of the conformers showed very small red-shifts of ~2 cm−1. In such cases coupled vibrations have been observed between O4−H6 and O3−H5 groups. In 1,4-BD the red-shifts of the two bands relative to the O−Hfree stretching mode observed at 3672.5 cm−1 are 124.1 and 66.0 cm−1 for the g′GG′Gt and gG′G′Gt conformers, respectively. It clearly indicates that the extent of IHB is different in these two conformers. It depends on the distance between accepter oxygen (OA) and the donor H atom involved in the hydrogen bond formation. This distance is calculated at the B3LYP/aug-cc-pVDZ level of theory to be 1.873 Å in g′GG′Gt conformer whereas in gG′G′Gt it is 2.098 Å as listedin Table 4. In 1,2-ED, it is difficult to get the hydrogen bond enthalpy for the individual conformers where the two hydroxyl groups are close to each other because bands with interacting and noninteracting hydroxyl groups are not well resolved in the experimental spectrum. Based on experimentally observed red-shift and equation (2), we have estimated the intramolecular interaction enthalpy of 1,2-ED to be −3.2 kJ mol−1 which is close to the interaction energy −3.3 kJ mol−1 estimated previously with the MPW1PW91/6-311+G(2d,p) method.20 On the other hand, in 1,4-BD, we could calculate the hydrogen bond enthalpy for two intramolecularly hydrogen bonded conformers since bands are well separated in the experimental spectrum. From Tables 2 and 3, we can see that the hydrogen bond enthalpy is more in 1,4-BD compared to that in 1,2-ED. In 1,4-BD, a seven member ring results from the formation of the IHB and energy of formation is maximum whereas in 1,2-ED the ambiguous IHB formation leads to a five-member quasi-ring whose strain energies nearly cancel out the stabilization energy. Another contribution 17 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 50

to the enhanced stabilization of the 1,4-BD lies in the possibility of closer approach of the two hydroxyl groups to each other, which is, in turn, reflected in the large O−H stretching frequency shift. From these data, we conclude that the intramolecular interaction or intramolecular hydrogen bonding is a major stabilizing factor in the gauche conformers of 1,4-BD. However, experimentally observed % population of conformers indicate that IHB conformers are less populated compared to the non-IHB conformers which clearly indicates that the contribution is less and so the relative stability is less.

4.4. Nature of hydrogen bond in diols. In general, upon hydrogen bond formation charge transfer occurs from the hydrogen acceptor (OA) to donor (OD−H) group and thus increase in the magnitude of the electrical transition dipole moment of the O−H stretching mode, which is the reason for the intensity enhancement. The formation of hydrogen bond also increases the O−H equilibrium distance which causes red-shift of O−H stretching frequency. The values calculated for structural characteristics related to the hydrogen-bonding manifestations for the lowest energy conformers of 1,2-ED and 1,4-BD are displayed in Table 4. In 1,4-BD, the formation of an intramolecular hydrogen bonded conformation involves the distortion of the carbon chain to a gauche arrangement which helps to bring two hydroxyl groups closer to each other. The OD−H (r) and OA⋅⋅⋅H (d) bond lengths and OA⋅⋅⋅H−OD (θ)angle (see Figure 3 for the definition of r, d, and θ) listed in Table 4 clearly show that the calculated shortest hydrogen bond donor-acceptor distance (OA⋅⋅⋅H), (1.9 − 2.1 Å) found in 1,4-BD is within the criteria defined for the existence of an IHB proposed by Desiraju and Steiner.44 However, the deviation of OA⋅⋅⋅H−OD angle from the most favorable geometry (180°) turns this bond into a weak one. In 1,2-ED, the calculated OA⋅⋅⋅H bond distance and OA⋅⋅⋅H−OD bond angle


Page 19 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lie around 2.4 Å and 106 – 110°, respectively, which is below the criteria of IHB.45 Apart from this bond distance and angle criteria, another criteria can be used for the assignment of hydrogen bond character is the “H-bond radii” as proposed by Raghavendra et al. in 2006.46 The OA⋅⋅⋅H distance in 1,2-ED, 2.4 Å is slightly less than the sum of van der Walls radii of O and H (2.6 Å), it is more than the sum of ‘hydrogen bond radii’ (2.0 Å), appropriate for OH (0.7 Å) and O (1.3 Å). However, conclusions from them are suspect because both the van der Waals ‘radii’ and hydrogen bond ‘radii’ do not account for the effect of anisotropy. In 1,2-ED, the small red-shift is found to be ~45 cm−1. This small red-shift could be due to intramolecular interactions between two hydroxyl groups. Our gas phase spectra analysis for 1,2ED preserves the gauche configuration in the gas phase at experimental temperature and a small red-shift in the O−Hin stretching frequencies without changing intensity. Electron topology analysis has not shown a BCP and atomic path for intramolecular hydrogen bonding raise its an important question: What is the driving force that maintains the gauche configuration in 1,2-ED in the gas phase if it is not primarily the O−H⋅⋅⋅O interaction as frequently assumed in the literature? The IUPAC hydrogen bond definition by Arunan et al. stated that BCP criteria is not mandatory for hydrogen bond.45 This has been supported by latest NCI calculations by Lane et al. and argued that the absence of a BCP should not necessarily be considered as evidence for absence of a chemical bond.32 Howard et al. reported experimental ∆ν red-shifts for the two most stable conformers of 1,2ED (tGg′ and gGg′) of 285 and 173 cm−1, respectively, in the fourth overtone vapor-phase spectra (∆νOH=5) and proposed that this red shift occur as a result of the weak O−H⋅⋅⋅O interaction.27 However, the red-shift within a conformer is not necessarily a measure of hydrogen bonding since the stretching frequency for the ‘free’ gauche O−H (O3−H5 group in Figure 1) in 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 50

the gGg′ is red-shifted by 208 cm−1 at ∆νOH=5 compared to the ‘free’ trans O−H group in the tGg′ conformer. The rotation of trans O−H group in 1,2-ED to either the g and g′ configuration with or without the possibility of interaction with the second O−H group, results in a ‘gauche’ red-shift of up to 30 − 40 cm−1 for the O−H fundamental vibrational wavenumber. This red-shift is increased by an increase in the σ*[O−H] occupancy. Even in strained cyclic diols, alarge redshift of 50 cm−1 has been found due to trans⇒gauche effect.31 This non-hydrogen bond related red-shifts result from O−H bond weakening due to σ⇒σ* interactions involving C−C and C−H bond but not from interactions with the oxygen lone-pair electrons. However, it is possible to see the IHB in the 1,2-ED in spite of absences of BCP at the equilibrium structure if there is a vibration that brings two O−H groups closer. Our measurement temperature is in the range of (303−323) K and at these temperature, thermal vibration of low frequency modes may help bring the two O−H groups close enough to be bonded. Recently, O−H⋅⋅⋅O interaction in 1,2-ED has been confirmed by Arunan et al. using AIM calculation; the involvement of low frequency O3⋅⋅⋅O4 stretching vibration for this interaction has been proposed.38 In case of 1,4-BD, the analysis of the natural bond orbitals (NBO) demonstrates the HB acceptor oxygen LP electrons to be of the highest energy and lowest energy occupancy for the LP with delocalization into the O−HD antibonding orbital.20,35 Reduction in occupancy for the oxygen LP associated with the increase in occupancy for the O−H antibonding orbital is consistent with the energy stabilization. The no⇒σ*[O−H] stabilization energy was calculated for g′GGG′t conformer with the MP2/6-311++G(d,p)35 and MPW1PW91/6-311+G(2d,p)23 method to be −43 and −48 kJ mol−1, respectively. The no⇒σ*[O−H] stabilization energy was calculated for tGg′ conformer in 1,2-ED with the MPW1PW91/6-311+G(2d,p) method to be ~2


Page 21 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kJ mol−1 which is far below compared to that in 1,4-BD.23 Based on these results, the IHB in 1,4BD with seven member ring is the most stable. However, from the observed gas phase spectra for 1,4-BD and equilibrium population analysis, it has been found that the % population of hydrogen bonded conformers are less compared to some of the non-IHB conformers. Perhaps, this is due to switching of the hydrogen bonded conformers to non-hydrogen bonded conformers at our experimental temperatures (313−333 K). In 1,2-ED, the intensity between O−Hin and O−Hout is enhanced by a factor of 1.7 for a mixture of conformers in the observed spectrum. From Table 2, we note that tGg′ is the most stable ambiguous hydrogen bonded conformer of 1,2-ED and it has shown the insignificant intensity enhancement in the O−Hin with respect to O−Hout stretching vibration. Although, the second most populated conformer (gGg′) has shown the intensity enhancement by a factor of 1.6, however, the extent of red-shift is very small ~32 cm−1 which is below 45 cm−1 red-shift observed for the tGg′ conformer. It indicates that the gGg′ conformer in 1,2-ED has less intramolecular hydrogen bond interaction compared to that in tGg′ conformer. On the other hand, in 1,4-BD, experimentally we found that there is a red-shift more than 65 cm−1 as well as significant intensity enhancement have been observed between O−Hbound and O−Hfree stretching vibrations in the intramolecularly hydrogen bonded conformers which is consistent with the calculation. Some of the conformers, as for example gGTGt, have shown a red-shift of 12−26 cm−1 and no intensity enhancement in the calculation. For this conformer there is no possibility of IHB formation. Usually upon hydrogen bond formation spectral broadening is seen in the O−H vibrational band along with frequency shift and IR intensity enhancement. This could be due to contribution



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 50

coming from homogeneous and/or inhomogeneous broadening.28,47 The spectral broadening in the O−Hin/bound has been evidenced from IR studies on alkanediols in CCl4 solution; the O−H stretching band width was reported to be 24.4 cm−1 for O−Hout and 34.1 cm−1 for O−Hin in 1,2ED36 and 22 cm−1 for O−Hfree and 78 cm−1 for O−Hbound in 1,4-BD.34 From Table 1, we can see that band width for O−Hout and O−Hin in 1,2-ED are found to be 42.9 and 41.1 cm−1, respectively, and 27.8 cm−1 and 25.2 cm−1 for O−Hfree and O−Hbound in 1,4-BD. From our gas phase studies, it is clear that the O−Hin/bound band-width is slightly lower than the O−Hout/free which is contrary to what is believed in the literature and observed in the solution phase IR spectra of diols. The greater band width in O−Hout/free stretching vibrational mode in observed spectra of 1,2-ED and 1,4-BD is due to the contribution of O−H groups from various conformers with different peak positions, within the range of (30 − 40) cm−1. From gas phase IR spectra of diols, we were unable to extract the O−Hout/free band-width for the hydrogen bonded conformers.

5. CONCLUSION We have reported the gas phase IR spectra of 1,2-ED and 1,4-BD at three different temperatures. The conformers of 1,2-ED and 1,4-BD have been investigated by means of DFT calculation and experimental methods. The infrared spectra of 1,2-ED and 1,4-BD in the gas phase are found to show a good agreement with a population weighted to calculated spectrum. Comparison between the observed and calculated spectra of diols reveals the existence of more than one conformers weighted by the Boltzmann factor in the gas phase. The small red-shift in O−Hin band position and no significant intensity enhancement compared to the O−Hout peak support the idea of weak intramolecular hydrogen bonding in 1,2-ED. On the other hand, in 1,4BD there is a 124 cm−1 shift in the O−H stretching frequency and intensity enhancement by a


Page 23 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


factor of 8.5 (Expt.) for the ‘bonded’ O−H compared to the ‘free’ O−H which support the existence of strong intramolecular hydrogen bonding in the compound. We have found that most stable hydrogen bonded conformers of 1,4-BD are less populated than some of the non-hydrogen bonded conformers which clearly indicates that the intramolecular interaction may not be a key factor for the most stable gauche conformer in diols. For the gas phase conformational analysis of 1,2-ED and 1,4-BD, Dunning’s correlation consistent basis set (aug-cc-pVDZ) in conjunction with DFT(B3LYP) method is found to be superior than the Pople’s basis set (6-311++G**).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 50

Figure 1. The 10 unique conformers of 1,2-ED. The structural degeneracy of each conformer and B3LYP/6-311++G** calculated relative energy (in kJ mol−1) w. r. t. stable conformer tGg′ are listed in parentheses. One of the conformers, 9 (tGt) does not belong to a stationary point at the B3LYP/6-311++G** level of theory.


Page 25 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Geometries of the 10 most populated conformers of each of the backbone families of 1,4-BD. The structural degeneracy of each conformer and B3LYP/6-311++G** calculated relative energy (in kJ mol−1) w. r. t. stable conformer (g′GG′Gt) are listed in parentheses.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 50

Figure 3. (a) Geometrical parameters (r, d, and θ) correspond to hydrogen bond are depicted here, X−H and A is the hydrogen bond donor and acceptor unit, respectively. (b) The O−Hin/out in 1,2-ED corresponds to the inward O−H group and outward O−H group in the tGg′ and gGg′ conformers. (c) Definition of hydrogen bonded O−H group (O−Hbound) and free O−H group (O−Hfree) in g′GG′Gt conformer of 1,4-BD; similar definition used for other hydrogen bonded conformer gG′G′Gt.


Page 27 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Experimental IR absorption spectrum of 1,2-ED in the gas phase at 303 K (A) and stimulated spectrum of a mixture of stable conformers tGg′, gGg′, g′Gg′, tTt, tTg, gTg′, gTg, gGg, tGt, and tGg (B). B3LYP/aug-cc-pVDZ calculated anharmonic intensities in the individual spectra of conformers were weighted by the population of the respective conformer as described in Table S1. The simulated spectrum (B) was obtained using Gaussian functions centered at the anharmonic frequencies and with a bandwidth at half-height of 35 cm−1. The O−H stretching region marked with gray.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 50

Figure 5. Experimental IR absorption spectrum of 1,4-BD in the gas phase at 313 K (A′), stimulated spectrum of a mixture of conformers g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt (B′). B3LYP/aug-cc-pVDZ calculated anharmonic intensities in the individual spectra of conformers were weighted by the population of the respective conformer as described in Table S2. The simulated spectrum (B′) was obtained using Gaussian functions centered at the anharmonic frequencies and with a band width at half-height of 35 cm−1. The O−H stretching region marked with gray.


Page 29 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Enlarged view of the O−H stretching region marked with gray in Figures 4 and 5 of observed and simulated spectra of 1,2-ED and 1,4-BD. Simulated spectra obtained with the B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ calculated anharmonic vibrational wavenumbers, anharmonic IR intensities and % population are shown by red and blue, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 30 of 50

Table 1. Vibrational wavenumber (ν, cm−1), FWHM (ν1/2, cm−1), integrated band area (S in cm−1) and ratio of integrated band area (R = Sin/Sout for 1,2-ED and R = Sbound/Sfree for 1,4-BD) of O−H absorption bands observed at different temperatures. Compound

1,2-ED

1,4-BDb

a

T/K

O−Hout/free ν1/2 42.3

a

S 1.947

ν 3637.5

O−Hin/bound ν1/2 41.6

R S 2.582

1.326

303

ν 3682.8

313

3682.2

45.3

3.036

3637.3

41.3

3.551

1.169

323

3681.1

41.2

3.873

3637.7

40.3

4.959

1.280

313

3672.5

27.2

0.272

3606.5

8.6

8.6×10−3

0.032

3548.4

24.0

0.288

1.059

3606.1

11.3

2.2×10−2

0.047

3548.7

25.0

0.475

1.008

3605.9

15.9

4.9×10−2

0.057

3549.2

26.6

0.799

0.930

323

333

3672.5

3672.5

27.7

28.6

0.471

0.859

a

In 1,2-ED, two O−H stretching vibrations are defined as O−Hin and O−Hout. In 1,4-BD they are defined as O−Hbound and O−Hfree. See Figure 3 for the definition of different type of O−H groups. b In 1,4-BD, bands observed at 3606 and 3548 cm−1 for the bound O−H corresponds to two hydrogen bonded conformers, see section 4.2 for the assignment.


Page 31 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 2. B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ calculated O−H stretching anharmonic vibrational wavenumbers and their anharmonic IR intensities (in km mol−1), observed (at 303 K) O−H stretching vibrational wavenumbers (in cm−1) and their band areas (in cm−1), red shifts (in cm−1) between ‘in’ and ‘out’ O−H stretching frequencies, and H-bond enthalpy (in kJ mol−1) in 1,2-ED. Conformers of 1,2-ED

O4−H6in

shift

O3−H5out

6-311++G**

aug-cc-pVDZ

6-311++G**

aug-cc-pVDZ

Freq. (Int.)

Freq. (Int.)

Freq. (Int.)

Freq. (Int.)

(cm−1)

tGg′

3636.1 (34.3)

3605.8 (31.5)

3658.9 (35.3)

3653.2 (33.1)

47.4

gGg′

3600.4 (36.6)

3586.1 (33.5)

3645.2 (29.0)

3618.0 (21.2)

31.9

g′Gg′

3682.2 (15.7)c

3638.0 (6.2)

3684.2 (56.8)c

3635.8 (39.0)

tTt

3704.5 (0.0)

3664.4 (0.0)

3705.3 (66.6)

3665.0 (64.1)

tTg

3644.7 (21.6)

3630.5 (20.5)

3664.2 (29.6)

3651.2 (28.9)

gTg′

3630.8 (46.9)

3628.3 (45.2)

3632.3 (0.0)

3629.2 (0.0)

gTg

3630.9 (21.9)

3620.3 (24.1)

3631.6 (16.8)

3621.2 (15.7)

gGg

3656.5 (30.1)

3619.6 (26.2)

3656.7 (16.9)

3619.9 (17.0)

tGt

--

3664.2 (17.0)

--

−∆Ha

Rel.

kJ mol−1

Int.

3.8

0.9/1.2b 1.6/1.6b

3664.3 (42.2)

tGg

3650.8 (15.6)

Gas phase

3637.5

d

2.6

3682.8

d

1.5

45.3

3.2

Reportedf

3644.0

--

3677.0

--

33.0

--

--

Reportedg

3607.0

--

3642.8

--

35.6

--

--

3623.2 (15.8)

3665.2 (27.1)


3640.6 (27.3) 1.7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Reportedh

3642.1

--

3663.2

a

Page 32 of 50

--

39.1

--

--

Estimated using the Iogansen empirical relation (2) based on red-shift. Calculated with the QCISD/6-311++G(2d,2p) level of theory.37 c B3LYP/6-311++G** calculated scaled harmonic frequencies and infrared intensities. Frequencies in the O−H stretching region are scaled by a factor of 0.9578. d Observed band area for O−H stretching vibration in 1,2-ED is listed in lieu of intensity since the exact concentration is not known. It was obtained from the band area reported in Table 1. e For O−Hout, this value was obtained from Table 1 after weighing it by the percent populations corresponding to conformers tGg′ and gGg′. f Data taken from the gas phase IR measurement at 45 oC.14 g Data taken from an IR spectroscopic study in CCl4.36 h Data taken from Ar matrix isolation IR spectroscopic study.43 b


Page 33 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 3. B3LYP/6-311++G** and B3LYP/aug-cc-pVDZcalculated O−H stretching anharmonic vibrational wavenumbers and their anharmonic IR intensities (in km mol−1), observed (at 313 K) O−H stretching vibrational wavenumbers (in cm−1) and their band areas (in cm−1), red shifts (in cm−1) between ‘free’ and ‘bound’ O−H stretching frequencies, and H-bond enthalpy (in kJ mol−1) in 1,4-BD. Conformers of 1,4-BD

O5−H7bound

shift

O6−H8free

6-311++G**

aug-cc-pVDZ

6-311++G**

aug-cc-pVDZ

Freq. (Int.)

Freq. (Int.)

Freq. (Int.)

Freq. (Int.)

(cm−1)

−∆Ha

Rel.

kJ mol−1

Int.

g′GG′Gt

3503.6(256.1)

3470.7 (258.3)

3674.3(31.3)

3648.0 (30.8)

177.3

16.2

8.4/9.8b

gG′G′Gt

3582.6(96.4)

3568.7(86.3)

3663.5(28.6)

3640.2 (27.2)

71.3

7.7

3.2

tG′TGt

3665.5(51.1)

3684.5 (48.5)

3665.6(0.0)

3684.5 (0.0)

g′TTGt

3622.1(17.7)

3619.3 (17.4)

3682.3 (28.6)

3655.9 (26.9)

gGTGt

3669.2 (17.7)

3623.8 (16.7)

3656.5 (15.1)

3649.9 (25.6)

tTTTt

3691.6(50.5)

3658.2 (33.8)

3691.1(0.0)

3657.9 (0.0)

tGGGt

3662.4(29.5)

3648.0(21.1)

3662.6(18.6)

3647.9 (14.0)

tTGG′t

3659.9 (24.3)

3641.7 (22.9)

3648.0 (22.3)

3649.3 (22.3)

gTGGt

3645.5(17.3)

3631.0 (16.7)

3676.6(28.4)

3652.8 (27.0)

tTGTt

3648.4(32.9)

3648.2(34.5)

3648.7(18.0)

3648.6 (12.5)

Gas phase

3548.4

c

2.9×10−1

3606.5

c

8.6×10−3

Reportede

--

3672.5

--

d

3.5×10−2

124.1

12.7

8.5

d

1.2×10−2

66.0

7.1

0.7

--

110.0

11.6

--


--


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Reportedf

3474.1

--

3634.3

a

Page 34 of 50

--

160.0

15.2

--

Estimated using the Iogansen empirical relation (2) based on red-shift. Calculated with the QCISD/6-311++G(2d,2p) level of theory.37 c Observed band area in cm−1. d Observed band area at 3672.5 cm−1 for the free O−H group has a contribution from all the conformers chosen for the calculation. Therefore, observed band area for the hydrogen bonded conformers is obtained by weighted percent population of the total band area at 3672.5 cm−1. e Red-shift value taken from ref. 15. f Data taken from an IR spectroscopic study in CCl4.34 b


Page 35 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ calculated hydrogen bond parameters listed before and after slash (/), respectively, for lowest energy conformers in 1,2-ED and 1,4BD. Hydrogen bonding parametersa Compound Conformer

1,2-ED

OA⋅⋅⋅H /Å

∠OA⋅⋅⋅H−OD

tGg′

2.395/ 2.409

0.964/ 0.967

106.4/ 106.2

gGg′

2.397/ 2.401

0.965/ 0.967

109.6/ 109.7

tTtb

1,4-BD

OD−H /Å

0.961/ 0.963

g′GG′Gt

1.874/ 1.873

0.970/ 0.973

155.9/ 156.5

gG′G′Gt

2.063/ 2.098

0.967/ 0.968

141.6/ 141.1

tTTTtb

0.961/ 0.964

a

OD = donor oxygen: atom number 4 in 1,2-ED (see Figure 1) and 5 in 1,4-BD (see Figure 2); OA = acceptor oxygen: atom number 3 in 1,2-ED (see Figure 1) and 6 in 1,4-BD (see Figure 2). b One of the non-hydrogen bonded conformers, tTt in 1,2-ED and tTTTt in 1,4-BD listed in the table for the comparison of OD−H distance with hydrogen bonded conformers.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 50

Supporting Information Gas phase IR spectra of 1,2-ED and 1,4-BD at three different temperatures (Figure S1). Experimental IR absorption spectra of 1,2-ED and 1,4-BD in the gas phase and stimulated spectra of a mixture of stable conformers tGg′, gGg′, g′Gg′, tTt, tTg, gTg′, gTg, gGg, and tGg in 1,2-ED and a mixture of conformers g′GG′Gt, gG′G′Gt, tG′TGt, g′TTGt, gGTGt, tTTTt, tGGGt, tTGG′t, gTGGt, and tTGTt in 1,4-BD (Figure S(2-3)). Simulated spectrum of a mixture 64 conformers of 1,4-BD at the B3LYP/aug-cc-pVDZ level of theory (Figure S4). Tables (S1-S2) listing B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ calculated relative energies (in kJ mol−1) and equilibrium gas phase percentage populations of the conformers in 1,2-ED and 1,4-BD at three different temperatures of the conformers in 1,4-BD. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Phone: 91-80-2293-2582, Fax: 91-80-2360-1552

ACKNOWLEDGMENT The spectrometer used in the experiment was supported by the FIST program of the Department of Science and Technology, Government of India. We thank CSIR, Govt. of India for funding and Shubhadip Chakraborty of for his help during the experiments and DFT calculations.


Page 37 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Jonsdottir, S. O.; Klein, R. A. UNIQUAC Interaction Parameters for Molecules with –OH Groups on Adjacent Carbon Atoms in Aqueous Solution Determined by Molecular Mechanics−Glycols, Glycerol and Glucose. Fluid Phase Equilib. 1997, 132, 117−137. (2) Patel, R. N.; Banerjee, A.; Pendri, Y. R.; Liang, J.; Chen, C.-P.; Mueller, R. Preparation of a Chiral Synthon for an HBV Inhibitor:Enzymatic Asymmetric Hydrolysis of (1α,2β,3α)-2(benzyloxymethyl)cyclopent-4-ene-1,3-diol Diacetate and Enzymatic Asymmetric Acetylation of (1α,2β,3α)-2-(benzyloxymethyl)cyclopent-4-ene-1,3-diol. Tetrahedron: Asymmetry 2006, 17, 175−178. (3) Wang, X.; Wang, L.; Li, H.; Tang, X.; Chang, F.–C. Syntheses of Poly(ethylene oxide) Polyurethane Ionomers. J. Appl. Polym. Sci. 2000, 77, 184−188. (4) Bako, I.; Grosz, T.; Palinkas, G.; Bellissent-Funel, M. C. Ethylene Glycol Dimers in the Liquid Phase : A Study by x-ray and Neutron Diffraction. J. Chem. Phys. 2003, 118, 3215−3221. (5) Chopra, D.; Row, T. N. G.; Arunan, E.; Klein, R. A. Crystalline Ethane-1,2-diol does not have Intra-Molecular Hydrogen Bonding: Experimental and Theoretical Charge Density Studies. J. Mol. Struct. 2010, 964, 126−133. (6) Tratteberg, M.; Hedberg, K. Structure and Conformations of 1,4-Butanediol: ElectronDiffraction Evidence for Internal Hydrogen Bonding. J. Am. Chem. Soc. 1994, 116, 1382−1387. (7) Kazerouni, M. R.; Hedberg, L.; Hedberg, K. Conformational Analysis. 21. Ethane-1,2-diol. An Electron-Diffraction Investigation, Augmented by Rotational Constants and ab Initio 37 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 50

Calculations, of the Molecular Structure, Conformational Composition, SQM Vibrational Force Field, and Anti-Gauche Energy Difference with Implications for Internal Hydrogen Bonding. J. Am. Chem. Soc. 1997, 119, 8324−8331. (8) Christen, D.; Coudert, L. H.; Suenram, R. D.; Lovas, F. J. The Rotational/Concerted Torsional Spectrum of the g′Ga Conformer of Ethylene Glycol. J. Mol. Spectros. 1995, 172, 57−77. (9) Christen, D.; Coudert, L. H.; Larsson, J. A.; Cremer, D. The Rotational-Torsional Spectrum of the g′Gg Conformer of Ethylene Glycol: Elucidation of an Unusual Tunneling Path. J. Mol. Spectros. 2001, 205, 185−196. (10) Muller, H. S. P.; Christen, D. Millimeter and Submillimeter Wave Spectroscopic Investigations into the Rotation-Tunneling Spectrum of gGg′ Ethylene Glycol, HOCH2CH2OH. J. Mol. Spectros. 2004, 228, 298−307. (11) Nakao, Y.; Sugeta, H.; Kyogoku, Y. Hydrogen Bonding of 2,4-Pentanediol and Related Molecules Studied by Infrared and 1H-NMR Spectroscopy. Spectrochim. Acta. 1986, 42A, 251−257. (12) Gallwey, F. B.; Hawkes, J. E.; Haycock, P.; Lewis, D. 1H NMR Spectra and Conformations of Propane-1,2-diol, Meso- and Racemic Butane-2,3-diols, and some Alditols in Non-aqueous Media. J. Chem. Soc., Perkin Trans 1990, 2, 1979−1985.


Page 39 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Petterson, K. A.; Stein, R. S.; Drake, M. D.; Roberts, J. D. An NMR Investigation of the Importance of Intramolecular Hydrogen Bonding in Determining the Conformational Equilibrium of Ethylene Glycol in Solution. Magn. Reson. Chem. 2005, 43, 225−230. (14) Buckley, P.; Giguere, P. A. Infrared Studies on Rotational Isomerism. I. Ethylene Glycol. Canad. J. Chem. 1967, 45, 397−407. (15) Fishman, E.; Chen, T. L. An Investigation of the Hydrogen Bonding Characteristics of Butanediols. Spectrochim. Acta 1969, 25A, 1231−1242. (16) Nagy, P. I.; Dunn III, W. J.; Alagona, G.; Ghio, C. Theoretical Calculations on 1,2Ethanediol. Gauche-Trans Equilibrium in Gas-Phase and Aqueous Solution. J. Am. Chem. Soc.

1991, 113, 6719−6729. (17) Cramer, C. J.; Truhlar, D. G. Quantum Chemical Conformational Analysis of 1,2Ethanediol: Correlation and Solvation Effects on the Tendency to Form Internal Hydrogen Bonds in the Gas Phase and in Aquesous Solution. J. Am. Chem. Soc. 1994, 116, 3892−3900. (18) Oie, T.; Topol, I. A.; Burt, S. K. Ab Initio and Density Functional Calculations on Ethylene Glycol. J. Phys. Chem. 1994, 98, 1121−1128. (19) Csonka, G. I.; Csizmadia, I. G. Density Functional Conformational Analysis of 1,2Ethanediol. Chem. Phys. Lett. 1995, 243, 419−428. (20) Klein, R. A. Hydrogen Bonding in Diols and Binary Diol-Water Systems Investigated Using DFT Methods. II. Calculated Infrared OH-Stretch Frequencies, Force Constants, and NMR



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemical Shifts Correlate with Hydrogen Bond Geometry and Electron Density Topology. A Reevaluation of Geometrical Criteria for Hydrogen Bonding. J. Comput. Chem. 2003, 24, 1120−1131. (21) Trindle, C.; Crum. P.; Douglass, K. G2(MP2) Characterization of Conformational Preferences in 2-Substituted Ethanols (XCH2CH2OH) and Related Systems. J. Phys. Chem. A

2003, 107, 6236−6242. (22) Mandado, M.; Grana, A. M.; Mosquera, R. A. Do 1,2-Ethanediol and 1,2-Dihydoxybenzene Present Intramolecular Hydrogen bond? Phys. Chem. Chem. Phys. 2004, 6, 4391−4396. (23) Klein, R. A. Hydrogen Bonding in Strained Cyclic Vicinal Diols: The Birth of the Hydrogen Bond. Chem. Phys. Lett. 2006, 429, 633−637. (24) Guvench, O.; MacKerell, Jr., A. D. Quantum Mechanical Analysis of 1,2-Ethanediol Conformational Energetics and Hydrogen Bonding. J. Phys. Chem. A 2006, 110, 9934−9939. (25) Kazuch, S.; Bachrach, S. M.; Martin, J. M. L. Conformational Equilibria in Butane-1,4-diol: A Benchmark of a Prototypical System with Strong Intramolecular H-bonds, J. Phys. Chem. A

2014, 118, 293−303. (26) Bachrach, S. M. Microsolvation of 1,4-Butanediol: The Competition Between Intra- and Intermolecular Hydrogen Bonding. J. Phys. Chem. A 2014, 118, 1123−1131.


Page 40 of 50

Page 41 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Howard, D. L.; Jorgensen, P.; Kjaergaard, H. G.Weak Intramolecular Interaction in Ethylene Glycol Identified by Vapor Phase OH−Stretching Overtone Spectroscopy. J. Am. Chem. Soc. 2005, 127, 17096−17103. (28) Cheng, Y.-L.; Chen, H.-Y.; Takahashi, K. Theoretical Calculation of the OH Vibrational Overtone Spectra of 1-n Alkane Diols (n = 2−4): Origin of Disapparing Hydrogen-Bonded OH Peak. J. Phys. Chem. A 2011, 115, 5641−5653. (29) Klein, R. A. 4Ab Initio Conformational Studies on Diols and Binary Diol-Water Systems Using DFT Methods. Intramolecular Hydrogen Bonding and 1:1 Complex Formation with Water. J. Comput. Chem. 2002, 23, 585−599. (30) Klein, R. A. Electron Density Topological Analysis of Hydrogen Bonding in Glucopyranose and Hydrated Glucopyranose. J. Am. Chem. Soc. 2002, 124, 13931−13937. (31) Crittenden, D. L.; Thompson, K. C.; Jordan, M. J. T. On the Extent of Intramolecular Hydrogen Bonding in Gas-Phase and Hydrated 1,2-Ethanediol. J. Phys. Chem. A 2005, 109, 2971−2977. (32) Lane, J. R.; Contreras-Garcia, J.; Piquemal, J.-P.; Miller, B. J.; Kjaergaard, H. G. Are Bond Critical Points Really Critical fo Hydrogen Bonding? J. Chem. Theory Comput. 2013, 9, 3263−3266.

(33) Steiner, T. The Hydrogen Bond in the Solid State. Angew. Chem., Int. Ed. 2002, 41, 48−76.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 50

(34) Jesus, A. J. L.; Rosado, M. T. S.; Leitao, M. L. P.; Redinha, J. S. Molecular Structure of Butanediol Isomers in Gas and Liquid States: Combination of DFT Calculations and Infrared Spectroscopy Studies. J. Phys. Chem. A 2003, 107, 3891−3897. (35) Jesus, A. J. L.; Rosado, M. T. S.; Reva, I.; Fausto, R.; Eusebio, M. E. S.; Redinha, J. S. Structure of Isolated 1,4-Butanediol: Combination of MP2 Calculations, NBO Analysis, and Matrix-isolation Infrared Spectroscopy. J. Phys. Chem. A 2008, 112, 4669−4678. (36) Ma, X.; Wang, J. Differentiating Subtle Variation of Weak Intramolecular Hydrogen Bond in Vicinal Diols by Linear Infrared Spectroscopy. J. Phys. Chem. A 2009, 113, 6070−6076. (37) Howard, D. L.; Kjaergaard, H. G. Influence of Intramolecular Hydrogen Bond Strength on OH−Stretching Overtones. J. Phys. Chem. A 2006, 110, 10245−10250. (38) Arunan, E.; Mani, D. Dynamics of the Chemical Bond: Inter- and Intra-Molecular Hydrogen Bond. Faraday Discuss. 2014, DOI: 10.1039/c4fd00167b. (39) Das, P.; Arunan, E.; Das, P. K. Infrared Spectra of Dimethylnaphthalenes in the Gas Phase. Vibr. Spectrosc. 2008, 47, 1−9. (40) Das, P.; Manogaran, S.; Arunan, E.; Das, P. K. Infrared Spectra of Dimethylquinolines in the Gas Phase: Experiment and Theory. J. Phys. Chem. A 2010, 114, 8351−8358. (41) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al.. Gaussian 09, Revision D.01; Gaussian Inc.: Pittsburg, PA, 2013.


Page 43 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(42) Iogansen, A. V. Direct Proportionality of the Hydrogen Bonding Energy and the Intensification of the Stretching ν(XH) Vibration in Infrared Spectra. Spectrochim. Acta A 1999, 55, 1585−1612. (43) Park, C. G.; Tasumi, M. Reinvestigation of Infrared-Induced Conformational Isomerization of 1,2-Ethanediol in Low-Temperature Ar Matrices and Reverse Reaction in the Dark. J. Phys. Chem. 1991, 95, 2757−2762. (44) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond In Structural Chemistry and Biology; Oxford University Press: New York, 1999. (45) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; et al. Defining the Hydrogen Bond: An account (IUPAC Technical Report)*. Pure Appl. Chem. 2011, 83, 1619−1636. (46) Raghavendra, B.; Mandal, P. K.; Arunan, E. Ab Initio and AIM Theoretical Analysis of Hydrogen-Bond Radius of HD (D = F, Cl, Br, CN, HO, HS and CCH) Donors and Some Acceptors. Phys. Chem. Chem. Phys. 2006, 8, 5276−5286. (47) Takahashi, K. Theoretical Study on the Effect of Intramolecular Hydrogen Bonding on OH Stretching Overtone Decay Lifetime of Ethylene Glycol, 1,3-Propanediol, and 1,4-Butanediol. Phys. Chem. Chem. Phys. 2010, 12, 13950−13961.

TOC graphics 43 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 44 of 50

Page 45 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. The 10 unique conformers of 1,2-ED. The structural degeneracy of each conformer and B3LYP/6311++G** calculated relative energy (in kJ mol¯1) w. r. t. stable conformer tGg’ are listed in parentheses. One of the conformers, 9 (tGt) does not belong to a stationary point at the B3LYP/6-311++G** level of theory. 276x101mm (150 x 150 DPI)

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Geometries of the 10 most populated conformers of each of the backbone families of 1,4-BD. The structural degeneracy of each conformer and B3LYP/6-311++G** calculated relative energy (in kJ mol¯1) w. r. t. stable conformer (g’GG’Gt) are listed in parentheses. 279x160mm (150 x 150 DPI)


Page 46 of 50

Page 47 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) Geometrical parameters (r, d, and θ) correspond to hydrogen bond are depicted here, X–H and A is the hydrogen bond donor and acceptor unit, respectively. (b) The O–Hin/out in 1,2-ED corresponds to the inward O–H group and outward O–H group in the tGg’ and gGg’ conformers. (c) Definition of hydrogen bonded O–H group (O–Hbound) and free O–H group (O–Hfree) in g’GG’Gt conformer of 1,4-BD; similar definition used for other hydrogen bonded conformer gG’G’Gt. 185x88mm (150 x 150 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Experimental IR absorption spectrum of 1,2-ED in the gas phase at 303 K (A) and stimulated spectrum of a mixture of stable conformers tGg’, gGg’, g’Gg’, tTt, tTg, gTg’, gTg, gGg, tGt, and tGg (B). B3LYP/aug-cc-pVDZ calculated anharmonic intensities in the individual spectra of conformers were weighted by the population of the respective conformer as described in Table S1. The simulated spectrum (B) was obtained using Gaussian functions centered at the anharmonic frequencies and with a bandwidth at halfheight of 35 cm¯1. The O–H stretching region marked with gray. 215x215mm (300 x 300 DPI)


Page 48 of 50

Page 49 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Experimental IR absorption spectrum of 1,4-BD in the gas phase at 313 K (A’), stimulated spectrum of a mixture of conformers g’GG’Gt, gG’G’Gt, tG’TGt, g’TTGt, gGTGt, tTTTt, tGGGt, tTGG’t, gTGGt, and tTGTt (B’). B3LYP/aug-cc-pVDZ calculated anharmonic intensities in the individual spectra of conformers were weighted by the population of the respective conformer as described in Table S2. The simulated spectrum (B’) was obtained using Gaussian functions centered at the anharmonic frequencies and with a band width at half-height of 35 cm¯1. The O–H stretching region marked with gray. 215x215mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Enlarged view of the O–H stretching region marked with gray in Figures 4 and 5 of observed and simulated spectra of 1,2-ED and 1,4-BD. Simulated spectra obtained with the B3LYP/6-311++G** and B3LYP/aug-cc-pVDZ calculated anharmonic vibrational wavenumbers, anharmonic IR intensities and % population are shown by red and blue, respectively. 228x203mm (300 x 300 DPI)


Page 50 of 50

Conformational Stability and Intramolecular Hydrogen Bonding in 1, 2

Recommend Documents