Intramolecular Hydrogen Bonding in Methyl Lactate - The Journal of

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Intramolecular Hydrogen Bonding in Methyl Lactate Sidsel Dahl Schroder, Jens Heide Wallberg, Jay A. Kroll, Zeina Maroun, Veronica Vaida, and Henrik G. Kjaergaard J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b04812 • Publication Date (Web): 21 Aug 2015 Downloaded from http://pubs.acs.org on September 1, 2015

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The Journal of Physical Chemistry

Intramolecular Hydrogen Bonding in Methyl Lactate Sidsel D. Schrøder,† Jens H. Wallberg,† Jay A. Kroll,‡ Zeina Maroun,† Veronica Vaida,‡ and Henrik G. Kjaergaard∗,† †Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States and Cooperative Institute for Research in Environmental Sciences, Boulder, Colorado 80309, United States E-mail: [email protected] Phone: +45-35320334. Fax: +45-35320322

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Abstract The intramolecular hydrogen bonding in methyl lactate was studied with FTIR spectroscopy, intracavity laser photoacoustic spectroscopy and cavity ring-down spectroscopy. Vapor phase spectra were recorded in the ∆vOH = 1 - 4 OH-stretching regions and the observed OH-stretching transitions were compared with theoretical results. Transition frequencies and oscillator strengths were obtained using a onedimensional anharmonic oscillator local mode model with potential energy and dipole moment surfaces calculated at the CCSD(T)-F12a/VDZ-F12 level. The three most abundant conformers of methyl lactate all appear to possess an intramolecular hydrogen bond, with the hydroxyl group forming a hydrogen bond with either the carbonyl or ester oxygen. The intramolecular hydrogen bonds were investigated theoretically by analyses based on electron density topology, natural bond orbital analysis and visualization of the electrostatic potential energy.


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Introduction Hydrogen bonds are important for the physical and chemical properties of many biological systems. In order to understand how hydrogen bonds mediate dynamical processes or stabilize static structures, it is important to investigate the competition between different hydrogen bonds (intramolecular as well as intermolecular). For example, the relative strength of hydrogen bonds strongly influences the structure and functionality of many biological molecules including DNA, enzymes, proteins, etc. 1,2 In the atmosphere, hydrogen bonds have an influence on important processes such as cloud- and aerosol formation, atmospheric photochemistry, and absorption of solar radiation. 3–9 It is well-known that electrostatic forces, dispersion forces, and forces associated with charge transfer from the hydrogen bond donor to the hydrogen bond acceptor contribute to the strength of a hydrogen bond. 10–12 The interplay between these forces is yet to be fully elucidated. For this purpose, the ability to characterize hydrogen bonds experimentally and theoretically is essential. A list of hydrogen bond criteria and characteristics has been published in the International Union of Pure and Applied Chemistry (IUPAC) definition of the hydrogen bond. 10 The IUPAC definition states that experimental signatures usually associated with hydrogen bonding include red shifts of vibrational transition frequencies. In addition, upon intermolecular complexation, intensities of hydrogen bonded stretching modes are often observed to change due to changes in the dipole moment function. 10,12–16 The theoretical characterization of hydrogen bonds is less well established and the criteria for correctly predicting hydrogen bonding have been debated extensively. 17–23 In the IUPAC recommendation of a definition of the hydrogen bond, it is stated that a hydrogen bond often is identified theoretically from the set of criteria proposed by Koch and Popelier based on an topological analysis of the electron density. 24 However, the adequacy of these hydrogen bond criteria has been debated in recent studies of intramolecular hydrogen bonded molecules. 22,23 It has been suggested that the non-covalent interaction (NCI) analysis 25 provides a more detailed picture of the intramolecular interactions. Methyl lactate (ML) is a relatively simple molecule which allows us to study the compe3 ACS Paragon Plus Environment


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tition between intramolecular hydrogen bonds as the hydroxyl group may form a hydrogen bond with either the carbonyl oxygen or the ester oxygen. As the two types of intramolecular hydrogen bonds contain identical atoms, ML is a useful model molecule for the investigation of the factors, other than atom type, which may influence hydrogen bond strengths. Due to its ability to form different kinds of inter- and intramolecular hydrogen bonds, ML has already been studied extensively. Previous studies suggest that at least two ML conformers with different intramolecular hydrogen bonds should be observable. 26–32 The OH· · · O=C hydrogen bonded ML conformer was observed in rotational spectra obtained with free jet millimeter-wave absorption and molecular beam Fourier transform microwave spectroscopy 26 as well as in experiments using pulsed molecular jet Fourier transform microwave spectroscopy. 27 Additionally, the OH· · · O=C hydrogen bonded ML conformer was observed in liquid phase studies using FTIR and vibrational circular dichroism spectroscopy. 28,29 Only a few studies have reported the observation of a second less stable conformer exhibiting an OH to divalent oxygen intramolecular hydrogen bond. Both conformers were identified in matrix isolation FTIR spectroscopy with argon and xenon matrices 30 and in liquid phase experiments conducted with vibrational circular dichroism and FTIR spectroscopy. 31 The presence of two conformers was also observed in the first OH-stretching overtone region of ML solvated in carbon tetrachloride with vibrational spectroscopy and circular dichroism. 32 Several studies have already focused on the competition beween intra- and intermolecular hydrogen bonds in ML. 33–38 In FTIR spectra of seeded supersonic jet expansions methanol was shown to disrupt the intramolecular hydrogen bond in ML and two other α-hydroxy esters. 33,34 The O18 labelled methanol was inserted into the intramolecular hydrogen bond, hereby favouring the cooperative O-HML · · · O-Hmethanol · · · O=CML hydrogen bond. 33 In liquid phase, the competition between intra- and intermolecular hydrogen bonding in ML was studied with combined vibrational absorption and vibrational circular dichroism spectroscopy. 34 The solvent effects on ML in carbon tetrachloride solution and methanol solution were investigated. 34 It was found that ML was mainly present as monomers and dimers in


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carbon tetrachloride solution whereas binary and quaternary ML-methanol complexes were dominant in methanol solution. 34 Hydrogen bonds are often studied with vibrational overtone spectroscopy where transition frequencies of different conformers become increasingly separated with higher vibrational excitation. 19,22,39,40 To the best of our knowledge, overtone spectra of vapor phase ML have not previously been recorded. However, gas phase overtone spectra of intramolecular OH· · · O=C hydrogen bonded conformers of glycolic acid, glyoxylic acid, 2,2-dihydroxyacetic acid and peroxyformic acid have been recorded with FTIR, photoacoustic and intracavity ring-down spectroscopy. 39,41–43 In the studies of glycolic acid and 2,2-dihydroxyacetic acid, conformers with intramolecular hydrogen bonds between O-H and divalent oxygen were predicted theoretically, but not observed. 39,42,43 Here we investigate the competition between the intramolecular hydrogen bonds in vapor phase ML monomer. We present spectra of the ML monomer in the ∆vOH = 1 - 4 OH-stretching regions obtained at room temperature using FTIR and intracavity laser photoacoustic spectroscopy (ICL-PAS). A combination of electronic structure calculations and anharmonic oscillator local mode model calculations guides the assignment of observed vibrational overtone transitions. 19,44 Transitions are assigned to the most abundant OH· · · O=C intramolecular hydrogen bonded ML conformer and to the less abundant OH to divalent oxygen intramolecular hydrogen bonded conformers of ML. Assignment of bands in the ∆vOH = 4 region is further aided by Birge-Sponer plots, ICL-PAS overtone spectra of ML-d1 (with hydroxyl hydrogen substituted with deuterium) and cavity ring-down (CRD) spectra obtained at two different temperatures. The experimental characterizations of the intramolecular hydrogen bonded conformers are complemented by theoretical analyses, which offer different theoretical perspectives on the hydrogen bonds. In atoms in molecules 24,45 (AIM) and non-covalent interactions (NCI) analyses 25 hydrogen bonds are predicted on the basis of the topology of the electron density. The intramolecular hydrogen bonds are further assessed from a theoretical point of view by visualization of the electrostatic potential energy on the



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electron density isosurfaces 46 and by investigation of the stabilization energies associated with charge transfer from the hydrogen bond donor to the hydrogen bond acceptor in a natural-bond orbital (NBO) analysis. 47,48

Theory and Calculations Computational details: The abundant conformers of ML were identified by running a conformer search with Molecular Mechanics using the MMFF force field followed by optimization with the B3LYP/6-31+G(d) method within Spartan’10. 49,50 The conformers were further optimized with Gaussian09 51 using the ωB97X-D/aVTZ 52 (aVTZ = aug-cc-pVTZ) method in combination with the keywords int = ultraf ine and opt = verytight. Conformers were further optimized with Molpro2012.1 53,54 using the explicitly correlated coupled cluster method, CCSD(T)-F12a/VDZ-F12 55 with the geminal exponent was set to 0.9. 56 Optimization threshold criteria were set to: energy = 10−9 a.u., step size = 10−6 a.u. and gradient = 10−6 a.u. Threshold criteria for all CCSD(T)-F12a single-point calculations were set to: energy = 10−9 a.u., orbital = 10−8 a.u. and step size = 10−8 a.u. AIM analyses were run by AIM2000. 57,58 NCI analyses were carried out with NCIPLOT 59 and the NCI isosurfaces were visualized with the VMD program. 60 Wavefunction files used for input to AIM- and NCI analyses were generated by Gaussian09. The NBO analyses were carried out with Gaussian09. All AIM-, NCI- and NBO analyses used wavefunctions obtained at the ωB97X-D/aVTZ level of theory. See SI for further details on AIM, NCI and NBO methods. Electrostatic potential energies were calculated at points on the electron density isosurfaces using checkpoint files generated with default settings at ωB97X-D/aVTZ level using Gaussian09. The electron density surfaces are color coded according to the value of the electrostatic potential in the range from -4.4 a.u. to +4.4 a.u. One-dimensional local mode model: The vibrational OH-stretching transitions of the ML conformers were calculated with the anharmonic oscillator local mode model. 19,44


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The anharmonic oscillator local mode model describes the isolated OH-stretching vibration as an isolated Morse oscillator with energies of the vibrational levels given by 1 1 ω − (v + )2 ωx ˜ E(v)/(hc) = (v + )˜ 2 2

(1)

The local mode frequency and anharmonicity, ω ˜ and ωx, ˜ were calculated in terms of the second, third, and fourth order derivatives obtained from a potential energy curve of 15 points along the internal OH-stretching displacement coordinate. 19,61 The points of the potential energy curve were calculated by displacing the internal OH-stretching coordinate from -0.30 ˚ A to +0.40 ˚ A in steps of 0.05 ˚ A about equilibrium bond length. 62 The oscillator strength of a vibrational transition from the vibrational ground state, | 0 , to an excited vibrational state, | v was calculated by 63,64

f0→v = 4.702 × 10−7 cm D−2 × ν˜v0 | µ ~ v0 |2

(2)

where µ ~ v0 is the transition dipole moment in Debye, v | µ ~ | 0 . The transition dipole moment was determined from an expansion around equilibrium. The expansion was limited to sixth order. The coefficients of this expansion were determined by fitting a sixth order polynomial to the dipole moment calculated for the same geometries used for the potential energy. 62

Experimental Methods Methyl lactate (ML, (-)-Methyl L-lactate, ≥ 97%) was obtained from Sigma Aldrich. Deuterated ML-d1 was synthesized by mixing ML with excess methanol-d4 (Euriso-top, > 99.80% D, water < 0.03%) in a flask. After mixing the solution overnight the flask was placed in an ice water bath in order to lower the vapor pressure of ML. The flask was connected to a vacuum line and methanol was distilled off. This procedure was repeated three times.



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We did not obtain a fully deuterated sample of ML. Consequently, the spectra of ML-d1 included features from both ML and ML-d1 (see Figures S13 and 3). The measured vapor pressures of ML-d1 samples are therefore not representative of the actual ML-d1 pressures and were not used to obtain experimental oscillator strengths. To remove atmospheric trace gasses, several freeze-pump-thaw cycles were conducted before FTIR and ICL-PAS spectra were recorded of ML and ML-d1 . Room temperature (23 ◦C - 26 ◦C) FTIR spectra were recorded for vapor phase ML and ML-d1 in the ∆vOH = 1 - 2 and ∆vOD = 1 - 3 regions, respectively. The FTIR spectra were recorded at 1.0 cm−1 resolution with a Bruker Vertex 80 FTIR spectrometer fitted with a CaF2 beam splitter. Spectra in the fundamental region were recorded with a mid-infrared (MIR) light source in combination with a liquid nitrogencooled MCT (Mercury Cadmium Telluride) detector. The overtone regions were obtained with a near-infrared (NIR) light source and an InGaAs (Indium Gallium Arsenide) detector. All FTIR spectra were recorded using a 4.8 m path length multi-reflection gas cell (Infrared Analysis, Inc) fitted with KCl windows and gold coated mirrors. The vapor phase spectra of ML in the ∆vOH = 3 - 4 regions were recorded with ICLPAS at room temperature. A description of the ICL-PAS setup can be found in the SI. Surprisingly, methanol was observed in all ICL-PAS spectra of ML and ML-d1 . The signals from methanol were removed by recording spectra of methanol and subtracting these from the ICL-PAS spectra of ML as shown in SI (Figures S4, S5 and S6)). The spectral calibration of the ICL-PAS spectra was confirmed by recording ML with air and comparing with well known transition frequencies listed in the HITRAN database of molecular oxygen, the socalled O2 A band, and water lines (see Figures S1 and S2). 65 Details about the experimental settings are located in Tables S5 - S7. The samples were prepared on a glass vacuum line with sample flasks, cells, and pressure gauges connected by swagelok fittings. Gas phase ML and ML-d1 were transferred from the sample flask to the sample cell used for measurements by opening the valves on the vacuum line between the sample flask and to the sample cell and letting the vapor pressure of the


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sample equilibrate throughout the vacuum line and cells. When the vapor pressure was reached, the valves to the sample flask and sample cell were closed. The pressure displayed on the pressure gauge (Varian Model PCG-750) was used to calculate the experimental oscillator strength with Eq. 4. However, condensation onto the inside cell surfaces proved to be an issue. For FTIR spectra, band area were observed to decrease over time when repeated measurements were conducted on the same sample. For ICL-PAS, condensation of ML on the windows on the photoacoustic cell disabled the laser beam to pass through the cell on several occasions. As a result, the actual pressure inside the cells during a measurement is expected to be lower than what was displayed by the pressure gauge just when the sample was loaded on the vacuum line. The experimental oscillator strengths are therefore associated with a significant uncertainty and the experimental oscillator strengths must be regarded as upper limits. Only relative intensites are obtained with ICL-PAS. However, the full region between 10000 - 14200 cm−1 was obtained using different optics and the relative oscillator strengths of the ∆vOH = 3 and 4 transitions could therefore be determined by overlapping spectra recorded in the different regions with long-wave, mid-wave and short-wave optics as shown in Figure S22. Uncertainties contributing to the observed relative oscillator strengths include whether the spectra obtained with different optics have been correctly scaled relative to each other. Furthermore, uncertainty arises from the curve fitted areas used to determine relative intensities as the overlap of different transitions complicated the process of curve fitting. Finally, it is an intrinsic property of ICL-PAS that weak absorptions more than strong absorptions. However, the excellent overlap between the ICL-PAS spectra and CRD spectra (where this is not a problem) of ML in the ∆vOH = 4 region shown in Figure S3 indicates that this last issue is not a great source of discrepancy. The temperature dependence of vapor phase ML was investigated using cavity ring-down (CRD) spectroscopy to facilitate the assignment in the ∆vOH =4 region. The setup of the CRD has previously been described in detail elsewhere. 39,66,67 Briefly, a tunable dye laser



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(Northern Lights, NL-5-2 MF6) with a spectral bandwidth below 1 cm−1 was pumped by a pulsed Big Sky frequency doubled Nd:YAG laser. To record the ∆vOH = 4 overtone spectrum, mirrors centered at 755 nm and a LDS 751 laser dye were used. At each wavelength, 100 ringdowns were averaged and converted to the absorption coefficient (in cm−1 ) using the relationship L1 α= Ls c

1 1 − τ τ0

(3)

with L/Ls = 94/76 for this experimental setup, where L is the length of the cavity, Ls is the length over which the sample is present, c is the speed of light, and τ and τ0 is the ringdown time with and without the sample, respectively. The spectrum was collected with a stepsize of 0.20 nm (∼ 3 - 4 cm−1 ). The sample was flowed through the cell by bubbling 200 SCCM (standard cubic centimetres per minute) of helium through the liquid sample and mixing it with a flow of 0.300 SLPM (standard litres per minute) helium for a total helium flow of 0.500 SLPM. τ0 was obtained by fitting a second order polynomial to a measured scan without the sample and with the same total flow of helium. Each of the mirrors was purged with 0.500 SPLM of helium in order to keep the mirrors clean. The spectrum was recorded at room temperature (22 ◦ C) and at an elevated temperature (86 ◦ C). The increased temperature was obtained by heating the sample on a water bath and having the cavity wrapped with heating tape. Experimental values for band center positions (˜ νobs. ), full width at half maximum (Γ) and band areas were obtained from Lorentzian line shapes fitted to each recorded spectrum R using OriginPro8.1. 68 The fitted spectra are shown in SI. The fitted band area, Ad˜ ν , was employed to calculate the experimental oscillator strength, f , of the transition, given by 64

−9

f = 2.6935 × 10 (K

−1

Torr m cm)

T

R

Ad˜ ν pl

(4)

where T is the temperature, p is the pressure and l is the path length of the sample cell.


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Results and Discussion Geometries: Three unique ML conformers with abundances higher than 1% were identified in the conformer search. In line with previous work, the three conformers are labelled SsC, G0 sk 0 C and GskC in relation to the Newman projection nomenclature for the three dihedral angles 6 H-O-C-C, 6 O-C-C=O and 6 O=C-O-CH3 (S,s = syn, G = gauche, sk = skew and C = cis). 27,30 The three dihedral angles of the optimized conformers are listed in Table 1. Table 1: Dihedral Angles, OH Bond Lengths and OH· · · O Angles of Methyl Lactate Conformers Optimized at the CCSD(T)-F12a/VDZ-F12 Level of Theory. Parameter 6 6 6 6

H-O-C-C (◦ ) O-C-C=O (◦ ) O=C-O-C (◦ ) O=C-C-C (◦ )

R(O-H) (˚ A) 6 O-H· · · O (◦ )

SsC

G0 sk 0 C

GskC

0.9 -3.6 -0.1 118.6

44.0 152.6 -0.5 -82.5

-38.8 -156.2 0.5 -34.3

0.9662 119.2

0.9620 109.6

0.9614 107.4

Figure 1 shows the three optimized ML conformers which all structurally appear to be stabilized by an intramolecular hydrogen bond interaction.

SsC

G0 sk 0 C

GskC

Figure 1: Structures of methyl lactate conformers optimized with the CCSD(T)-F12a/VDZF12 method. The possible intramolecular hydrogen bond of the SsC conformer is located between the hydroxyl group and the carbonyl oxygen. For both the G0 sk 0 C and GskC conformers, which have very similar structures, the possible hydrogen bond is located between the hydroxyl 11 ACS Paragon Plus Environment


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group and the oxygen of the ester group. Table 1 lists O-H bond lengths and O-H· · · O angles of the three ML conformers. The longer O-H bond length and the more linear OH· · · O angle of the SsC conformer suggest that the SsC conformer could exhibit a stronger intramolecular hydrogen bond interaction than the G0 sk 0 C and GskC conformers. 10 Table 2 lists relative electronic energies, relative Gibbs free energies and corresponding Boltzmann populations calculated at CCSD(T)-F12a/VDZ-F12 level with thermal corrections obtained from ωB97X-D/aVTZ calculations. Table 2: Energies (in kJ/mol) and Boltzmann Distributions of Methyl Lactate Conformers. Conformer

∆E a ∆Gb F (%)c

SsC G0 sk 0 C GskC

0.0 9.8 9.3

0.0 10.4 10.3

97.0 1.5 1.5

a

Calculated at CCSD(T)-F12a/VDZF12 level of theory. b Calculated from ∆E with thermal corrections obtained at ωB97XD/aVTZ level of theory. c Calculated from ∆G with a temperature of 298 K.

With a calculated abundance of 97% at room-temperature the SsC conformer is by far the most stable conformer and only minor contributions from the G0 sk 0 C and GskC conformers are expected in the vibrational spectra. OH-stretching Transitions: Figure 2 shows the room-temperature vapor phase spectra of ML in the ∆vOH = 1 - 4 regions.


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Figure 2: Room-temperature vapor phase spectra of methyl lactate in the ∆vOH = 1 - 4 regions recorded with FTIR spectroscopy (∆vOH = 1 - 2 ) and ICL-PAS (∆vOH = 3 - 4 ). Methanol signals have been subtracted from the ICL-PAS spectra.



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In the spectra of the ∆vOH = 1 - 3 regions two separate bands are clearly visible. Assignment of the multiple bands observed in the ∆vOH = 4 region is discussed later. Based on the relative energies and local mode calculations of the three optimized ML conformers, the most abundant conformer, SsC conformer, is assigned to the strongest of the two bands which is red shifted relative to the weaker band assigned to comprise transitions of the less abundant G0 sk 0 C and GskC conformers. Table 3 shows a good agreement between the fundamental OH-stretching transition frequencies observed in this study with fundamental transition frequencies reported from previous studies of ML in chloroform solutions, in jet experiments and in matrix isolation studies. 30,31,33 Table 3: Comparison of Observed Fundamental OH-Stretching Transition Frequencies (˜ νobs. in cm−1 ) of Methyl Lactate. Study This study Argon matrix (FTIR), Ref. 30 Xenon matrix (FTIR), Ref. 30 Jet (FTIR), Ref. 33 Carbon tetrachloride solution (FTIR), Ref. 31 Carbon tetrachloride solution (VCDa), Ref. 31 a

SsC

G0 sk 0 C/GskC

3574 3554/3549/3543/3540 3542 3565 3555 3551

3644 3629/3626 3629/3621 3612 3615

Vibrational Circular Dicroism.

In the OH-stretching regions, the band assigned to the SsC conformer shows an asymmetry, which separates out into clearly distinguishable bands with increasing vibrational excitation. The origin of these extra bands is unknown, and while observed, the bands have not been assigned in previous studies. 30,31,33 We ran a VPT2 calculation on the SsC conformer at B3LYP/VDZ level of theory to investigate possible assignments. For the band assigned to the ∆vOH = 1 transition of SsC, we found that several combination bands of CH-stretching transitions with rocking/wagging modes of the carbon frame, and the first overtone of C=O stretching were located at frequencies close to the SsC ∆vOH = 1 transition. The total intensity of these transitions were calculated to be small (less than 5%) and are the likely cause of the small asymmetry. The asymmetry towards blue of the band 14 ACS Paragon Plus Environment

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assigned to the ∆vOH = 2 transition of SsC is more pronounced than for the fundamental transition. The two mode combinations of the VPT2 calculation does not provide any overtone or combination bands with transition frequencies higher than the ∆vOH = 2 transition frequency and the C=O overtone plus OH-stretching or CH-stretching lie either much too high or too low. However, the asymmetry may arise from a combination band of three modes which could be a combination of the OH-stretching mode plus CH-stretching plus one of the previously mentioned low frequency transitions. The experimental peak positions (˜ νobs ), full width at half maxima (Γ), and oscillator strengths (fobs ) are listed in Table 4 alongside calculated transition frequencies and Boltzmann weighted oscillator strengths for comparison, with non-weighted oscillator strengths in the SI.



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Table 4: Observed and CCSD(T)-F12a/VDZ-F12 Calculated OH-Stretching Transition Frequencies and Oscillator Strengths of Methyl Lactate. Observed

Calculated ν˜calc fcalc a (cm−1 )

νõbs (cm−1 )

Γ (cm )

3575

26

3647

25

6956 6956

30 1.6×10−7 63 1.3×10−7 26 1.5 ×10−8

7121

4.0×10−6 3575 1.4×10−5 3658 9.0×10−8 1.1×10−7 3667 1.1×10−7

10150 10233

51 1b 150 1.002b

10432

39

b

0.192

13135 13304 13439

100 138 199

0.027b 0.026b 0.013b

13573

58

b

0.010

13671 13874

126 170

0.017b 0.007b

a b c

fobs

−1

Conformer

∆vOH

SsC G0 sk 0 C GskC

1 1 1

6974

2.8×10−7

SsC

2

7148 7166

4.9×10−9 5.7×10−9

G0 sk 0 C GskC

2 2

10193

1c

SsC

3

10469 10497

0.013c 0.015c

G0 sk 0 C GskC

3 3

13242

0.075c

SsC

4

13621 13659

0.001c 0.001c

G0 sk 0 C GskC

4 4

Boltzmann weighted oscillator strengths. Relative intensities with the observed SsC ∆vOH = 3 transition set to 1. Relative intensities with the calculated Boltzmann weighted oscillator strength of the SsC ∆vOH = 3 transition set to 1.

Local mode calculations with the CCSD(T)-F12a/VDZ-F12 method have previously shown to provide transition frequencies of hydrogen bonded OH-stretching transitions in good agreement with experimental values. 69 However, comparison of calculated and observed transition frequencies shows that the calculated local mode transition frequencies are overestimated relative to the observed transition frequencies. Furthermore, transition frequencies become increasingly overestimated by the local mode calculations with increasing vibrational excitation. The calculated absolute oscillator strengths of the ∆vOH = 1, 2 and 4 transitions of SsC


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are within a factor of 3.5 from the experimental values. For the transitions arising from the G0 sk 0 C/GskC conformers, calculated oscillator strengths are both over- and under estimated relative to the observed oscillator strengths. We do not see a clear pattern in the discrepancy between calculated and observed oscillator strengths. We believe that there are different contributions which may cause the calculated oscillator strengths to be overestimated. First, the observed oscillator strengths should be regarded as upper limits as the actual pressures of ML during a measurement is diminished due to condensation on the inside cell surfaces. Second, it has previously been observed that observed that calculated oscillator strengths of intermolecular hydrogen bonded stretching modes are overestimated due to the lack of anharmonic coupling to hydrogen bond liberating modes in our one-dimensional anharmonic oscillator local mode model. 70 We speculate that this may also be the case for our intramolecular hydrogen bonded ML conformers. In addition, we believe that disagreement between observed and calculated oscillator strengths are partly due to difficulties with curve fitting the spectra (See Figures S7 - S10) which are complicated by the asymmetry towards the blue of the bands assigned to SsC. Assignment of Bands in the ∆vOH = 4 Region: In the ∆vOH = 4 region from ∆vOH = 4 and ∆vCH = 5 transitions are present (calculated CH-stretching transition frequencies and intensities are given in Table S9). 22 The calculated intensity of the ∆vCH = 5 transitions is about a third of the calculated ∆vOH = 4 intensity. The assignment of bands in the ∆vOH = 4 region has been aided by deuteration experiments and a spectrum recorded at an elevated temperature. The spectrum of deuterated ML, ML-d1 , in the ∆vOH = 4 region is shown in Figure 3.



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Figure 3: Photoacoustic spectra of ML and ML-d1 of the ∆vOH = 4 region recorded at 24◦ C. If ML was completely deuterated, the bands arising from ∆vOH = 4 transitions of ML would disappear facilitating assignment of the ∆vCH = 5 transitions. However, it was not possible to fully deuterate ML and the partly deuterated ML-d1 sample was employed to obtain the spectrum in Figure 3 (See SI for details of the spectral subtraction). Based on the ML-d1 spectrum it was not possible to conclusively assign the SsC and G0 sk 0 C/GskC conformers to the spectrum. However, the two bands with peak maxima at 13143 cm−1 and 13582 cm−1 bands are tentatively assigned OH-stretching transitions as the difference between these two wavenumbers are in good agreement with the calculated difference between the wavenumbers of the ∆vOH = 4 transitions of the SsC and G0 sk 0 C/GskC conformers. The assignment of bands in the ∆vOH = 4 region was further supported by recording the spectrum of ML in this region at different temperatures. Spectra were recorded at 22◦ C and 86◦ C with CRD spectroscopy. At room temperature, the ICL-PAS and CRD are in excellent agreement (see SI). When the temperature is increased from 22◦ C to 86◦ C the abundance of the dominating SsC conformer is expected to drop whereas the abundances of the G0 sk 0 C and GskC conformers are expected to increase. Figure 4 shows that the intensity of the band located around 13143 cm−1 decreases with increasing temperature whereas the intensity of 18 ACS Paragon Plus Environment

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the band located around 13582 cm−1 increases with increasing temperature.

Figure 4: Cavity ring-down spectra of methyl lactate vapor in the ∆vOH = 4 region at 22◦ C and 86◦ C. The observed temperature effects suggest the band at 13143 cm−1 can be assigned to the SsC conformer and the band located around 13582 cm−1 be assigned to the G0 sk 0 C and GskC conformers. The remaining bands are left almost unaffected by the temperature increase in support of the assignment of these bands to CH-stretching transitions which are similar for the three conformers. The plot of observed ν˜/v against v yield a linear Birge-Sponer plot from which the experimental local mode parameters can be determined. The Birge-Sponer plots of the transitions, assigned to SsC and G0 sk 0 C/GskC are shown in Figure 5 and the derived local mode parameters in Table 5.



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Figure 5: Birge-Sponer plots and linear fits of ∆vOH = 1 - 4 transitions of two methyl lactate conformers. ν˜ / v is plotted versus v, where ν˜ is the transition frequency and v is the vibrational quantum number.

Table 5: Experimental and Calculated Local Mode Harmonic Frequencies, ω ˜, and Anharmonicity, ωx, ˜ of Methyl Lactate. Experimentala Conformer SsC G0 sk 0 C GskC

ω ˜ (cm−1 ) 3766 ± 1 3810 ± 1

Calculateda

ωx ˜ (cm−1 )

ω ˜ (cm−1 )

ωx ˜ (cm−1 )

95.9 ± 0.3

3751 3827 3836

88.2 84.4 84.1

83.0 ± 0.3

a

Obtained from linears fit to the Birge-Sponer plots. Given with one standard deviation. b Calculated at CCSD(T)-F12a/VDZ-F12 level of theory.

The small uncertainties of the experimental local mode parameters indicate good linear fits to the Birge-Sponer plots and support the assignment of bands as depicted in Figure 2. The observed and CCSD(T)-F12a/VDZ-F12 calculated local mode parameters, ω ˜ and ωx, ˜ are presented in Table 5. Comparison of the experimental and calculated local mode parameters shows that the calculated harmonic frequency and anharmonicity are underestimated for the SsC conformer and overestimated for the G0 sk 0 C/GskC conformers. The lower harmonic 20 ACS Paragon Plus Environment

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frequency and the higher anharmonicity of the SsC conformer result in a red shift of the OHstretching transition frequencies of the SsC conformer relative to that of the G0 sk 0 C/GskC conformers. The red shift of transition frequencies is a well-known spectroscopic signature of hydrogen bonding and suggests a stronger hydrogen bond in the SsC conformer. 10 Theoretical Interaction Analyses: We have employed AIM, NCI, NBO analyses and electrostatic potential energies to assess the intramolecular interactions in the three ML conformers theoretically. More detailed descriptions of the theories and results of these analyses are located in SI. The AIM and NCI analyses are both based on the topology of electron density and can be employed to predict the existence and relative strengths of hydrogen bonds. However, the AIM analysis and the NCI analysis provides no insights to the forces which contributes to the strength of a hydrogen bond. According to the IUPAC definition of a hydrogen bond of 2011, the hydrogen bond is composed by different forces including electrostatic forces and charge transfer. 10,71 Therefore, examination of the distribution of electrostatic energy potential around the molecule and the NBO analysis of the charge transfer associated with the intramolecular hydrogen bonds of the different conformers are two approaches which can be employed to gain insight as to which factors affect the strength of hydrogen bonds. In AIM and NCI analyses, the bonding of a chemical system is described in terms of the topology of the electron density. 25,45 No (3,-1) bond critical points representing a hydrogen bond were identified in any of the three optimized ML conformers by the AIM analysis. As a result, no intramolecular hydrogen bonds are predicted by the AIM analysis according to the set of criteria proposed by Koch and Popelier. 24 In an NCI analysis, a 2D NCI plot of the reduced density gradient, s, versus the product of the sign of the second eigenvalue of the Hessian and the electron density, sign(λ2 )ρ, is produced. 25 Troughs indicate the presence of non-covalent interactions. Troughs with sign(λ2 )ρ < 0 represent attractive interactions whereas troughs with sign(λ2 )ρ > 0 represent repulsive interactions. The 2D NCI plots of the three ML conformers are shown in Figure



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6a.

(a)

(b)

Figure 6: (a) In each of the 2D NCI plots, the trough at sign(λ2 )ρ < 0 represents the hydrogen bond interaction. (b) Blue/turquoise 3D NCI isosurfaces visualize the hydrogen bond interactions in the 3D space of the conformer. The 3D NCI isosurfaces are generated for a cut-off value of 0.3 a.u. (horizontal line in 2D NCI plots) with a color scale in the range -0.04 a.u. < sign(λ2 )ρ < 0.04 a.u.

In Figure 6b, 3D NCI isosurfaces, which visualize the (sign(λ2 )ρ, s) points in 3D space of the molecule, are shown. We have included only the points with a reduced density below 0.3 a.u. (horizontal line in 2D NCI plot). The strength of an interaction depends on the magnitude of ρ and in the 2D NCI plots and 3D NCI isosurfaces a three color code is applied to differentiate between attractive interactions (blue), very weak non-covalent interactions (green) and repulsive interactions (red). For all three conformers, a blue/turquoise isosurface is present in the path of the predicted intramolecular hydrogen bond. The darkest shade of blue, corresponding to the strongest hydrogen bond, is found in the SsC conformer. The yellow shading adjacent to the blue corresponds to repulsive interaction arising from ring 22 ACS Paragon Plus Environment

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strain. The correlation of the electron density with hydrogen bond strength in NCI was used to investigate the geometry around the hydrogen bond in the three conformers. For each conformer, the value of the dihedral angle, 6

H-O-C-C, was changed stepwise around its

equilibrium, while the remaining vibrational degrees of freedom were frozen (see Figure 7a).

(a)

(b)

Figure 7: (a) Arrows and shaded OH bonds depict how the ML structures change with θ. (b) shows the value of ρ at the bottom of the left trough (Figure 6a). θ = 0◦ is arbitrarily defined as the equilibrium geometry. The hydrogen bond strength correlates with the magnitude of ρ.

At each new geometry, a 2D NCI plot was produced and the electron density was determined from the position of the left trough. In Figure 7b, the electron density of the attractive interaction is plotted versus θ, i.e. the change of 6 H-O-C-C. At θ = 0, it is clear that the SsC conformer has the highest density in agreement with the observations in Figure 6. Interestingly, it is seen that the maximum electron density of the G0 sk 0 C and GskC conformers,



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respectively, does not lie at their equilibrium structures, θ = 0. Rather, the maximum electron density for the G0 sk 0 C and GskC conformers are obtained when the dihedral angle is rotated ∼25◦ from equilibrium. This is likely due to the fact that the equilibrium structures are a balance between ring strain and hydrogen bonding. Electrostatic forces contribute to the strength of a hydrogen bond. 10 Therefore, electrostatic potential energy on the electron density isosurfaces have been calculated for the three ML conformers and are shown in Figure 8.

Figure 8: The electrostatic potential energy of ML conformers plottet on the electron density isosurfaces. Positive and negative electrostatic potential energies are represented by blue and red areas, respectively. Electropositive and electronegative regions are represented by blue and red areas, respectively. The electrostatic potential energy show that the surface around the carbonyl 24 ACS Paragon Plus Environment

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oxygen is more electronegative than that around the ester oxygen, which partly why the SsC conformer is the most abundant conformer. In the NBO analysis of ML, the hydrogen bonding interaction is represented by the charge transfer from the lone-pair natural bond orbitals of the hydrogen bond accepting oxygen to the anti-bonding sigma natural bond orbital along the O-H bond. 47,48 The stabilization energy gained from the hydrogen bond interaction, denoted E (2) , is a measure of the interaction strength. The results from the NBO analysis of the three ML conformers are shown in Table 6. Table 6: NBO Stabilization Energies, E (2) , of the Hydrogen Bond Charge Transfer Calculated at the ωB97X-D/aVTZ Level of Theory. Molecule

Conformera #Cb Interaction

Methyl lactate SsC c Methyl lactate G0 sk 0 C c Methyl lactate GskC c Glycolic acid GAd Ethylene glycol EG1e Ethylene glycol EG2e Propane diol PD1f Propane diol PD2f Butane diol BD1f Butane diol BD2f Methallyl carbinol MAC1g Allyl carbinol AC1g Aminoethanol 2Ah Aminopropanol 3Ah Water dimer H2 O·H2 Oi

2 2 2 2 2 2 3 3 4 4 2 2 2 3

O-H· · · O=C O-H· · · O O-H· · · O O-H· · · O=C O-H· · · O O-H· · · O O-H· · · O O-H· · · O O-H· · · O O-H· · · O O-H· · · C=C O-H· · · C=C O-H· · · N O-H· · · N O-H· · · O

a

E (2) (kJ/mol) 12.9 2.0 3.9 11.0 1.8 3.2 18.5 17.5 50.0 47.0 3.7 2.9 9.9 40.3 38.7

The conformer name as used in the article from which the initial geometry was obtained. b Number of carbon atoms separating the functional groups involved in the hydrogen bond. c This study. d Ref. 43 e Ref. 19 f Ref. 72 g Ref. 22 h Ref. 40 i Ref. 76



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For each conformer, a stabilizing hydrogen bonding interaction was observed. The stabilization energy of the hydrogen bond interaction is larger for the SsC conformer than for the G0 sk 0 C and GskC conformers, which, in agreement with experimental observations and the NCI analysis, confirms that the hydrogen bond of the SsC conformer is stronger than the hydrogen bonds of the G0 sk 0 C and GskC conformers. In the literature, red shifts of OH-stretching overtone transition frequencies have experimentally verified the existence of abundant intramolecular hydrogen bonded conformers of glycolic acid, ethylene glycol, propane diol, butane diol, methallyl carbinol, allyl carbinol, aminoethanol and aminopropanol. 19,22,23,40,43,72,73 For these conformers, intramolecular hydrogen bonds were only predicted by the AIM analysis for aminopropanol, propane diol and butane diol. However, hydrogen bonds were predicted for all conformers by NCI analyses. A comparison of the NBO calculated stabilization energies for these conformers are listed in Table 6. By comparison of the calculated stabilization energies, it appears that there are two factors which influence the magnitude of charge transfer associated with the hydrogen bonding interaction. First, as the number of methylene groups in the carbon chain in the diols and aminoalcohols increases, the NBO calculated stabilization energy increases. Intuitively, this makes sense as the extra methylene groups allow the conformers to adopt geometries more favourable for hydrogen bonding. In complexes, like for example the water dimer, it is generally accepted that the two water monomers are held together by an intermolecular hydrogen bond. 74,75 The NBO analysis predicts the hydrogen bonding interaction between the two water monomers in the water dimer has a stabilization energy of E (2) = 39 kJ/mol. From Table 6 it is clear that the flexible butanediol and aminopropanol have hydrogen bond stabilization energies even higher than the water dimer. 19,40 Second, it appears that the stabilization energy depends on the types of atoms involved in the hydrogen bond interaction. In this way, the stabilization energies of G0 sk 0 C and GskC are very similar to the stabilization energies of the two conformers of ethylene glycol. These four conformers are all intramolecular hydrogen bonded with an OH to divalent oxygen separated by a chain of


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two methylene groups. The stabilization energies of glycolic acid and SsC are also similar. Both conformers possess intramolecular O-H· · · O=C hydrogen bonds separated by a carbon chain of two methylene groups.

Conclusion We have investigated the competition between intramolecular hydrogen bonds to carbonyl oxygen and ester oxygen. Guided by one-dimensional anharmonic oscillator calculations at CCSD(T)-F12a/VDZ-F12 level, two types of intramolecular hydrogen bonded conformers were identified in vibrational room-temperature vapor phase spectra of methyl lactate (ML). Spectra were recorded in the ∆vOH = 1 - 4 regions with FTIR spectroscopy (∆vOH = 1 - 2), ICL-PAS (∆vOH = 3 - 4) and CRD (∆vOH = 4). The bands assigned to the SsC conformer, which exhibits an OH· · · O=C intramolecular hydrogen bond, were observed to be stronger and more red shifted relative to the bands assigned to the G0 sk 0 C/GskC conformers, which exhibits OH to divalent oxygen intramolecular hydrogen bonds. These observations suggest that the OH· · · O=C intramolecular hydrogen bond is stronger and favoured over the OH to divalent oxygen intramolecular hydrogen bond. Assignment of the conformers in the ∆vOH = 4 region was facilitated by CRD spectra of ML recorded at two different temperatures and ICL-PAS spectra of deuterated ML. Theoretically, the AIM analysis did not predict any intramolecular hydrogen bonds. However, both the NCI analysis and the NBO analysis predicted the intramolecular OH· · · O=C hydrogen bond interaction of the SsC conformer to be stronger than that between OH and divalent oxygen in the G0 sk 0 C and GskC conformers in good agreement with the experimental observations. The NCI analysis revealed that at equilibrium geometries, the G0 sk 0 C and GskC conformers are not oriented in a position that is optimal for intramolecular hydrogen bonding. As opposed to this, the dominant SsC conformer has an equilibrium structure, which corresponds to maximal hydrogen bonding interaction. The magnitudes of NBO sta-



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bilization energies were observed to correlate with the hydrogen bond strength in a series of OH· · · O intramolecular hydrogen bonds.

Associated Contents Supporting Information: Photoacoustic spectra of methyl lactate recorded with O2 and H2 O used for spectral calibration in the ∆vOH = 4 region. Discussion of the appearance of methanol in the photoacoustic spectra. Illustration of the subtraction procedure of methanol from the photoacoustic spectra of methyl lactate. Spectra curve fitted with Lorentz line shapes. Spectra recorded of deuterated methyl lactate. Local mode calculated transition frequencies and oscillator strengths of methyl lactate and deuterated methyl lactate. Experimental settings and conditions used to record the vibrational spectra. Description of the experimental setup of the photoacoustic spectrometer. Descriptions of the theoretical interaction methods AIM, NCI and NBO. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements We thank Christian R. Parker, Erik Daa Funder and Nicolai Bork for help regarding the ML-d1 synthesis and Anne S. Hansen and Kasper Mackeprang for helpful discussions. We acknowledge the financial support from The Danish Council for Independent Research Natural Sciences, the Danish Center for Scientific Computing and MSF CHE-BO6386. Jay A. Kroll acknowledges support from the Eklay fellowship, administered by the Department of Chemistry and Biochemistry, University of Colorado. Zeina Maroun acknowledges support from the foundation of Hotel owner Anders M˚ ansson and wife.


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Intramolecular Hydrogen Bonding in Methyl Lactate - The Journal of

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