Intramolecular Interactions in 2-Aminoethanol and 3-Aminopropanol

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Intramolecular Interactions in 2-aminoethanol and 3-aminopropanol Ditte Linde Thomsen, Jessica Lynne Axson, Sidsel D Schrøder, Joseph R. Lane, Veronica Vaida, and Henrik Grum Kjaergaard J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp405512y • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 7, 2013

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The Journal of Physical Chemistry

Intramolecular Interactions in 2-aminoethanol and 3aminopropanol Ditte L. Thomsen,a Jessica L. Axson,b,c,† Sidsel D. Schrøder,a Joseph R. Lane,d Veronica Vaida,b,c and Henrik G. Kjaergaard a,*

a

Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100

Copenhagen Ø, Denmark. b

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-

0215, USA c

CIRES, Boulder, CO 80309, USA

d

Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton 3240, New

Zealand

ACS Paragon Plus Environment

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KEYWORDS Hydrogen bond, overtone spectrum, local mode, IR spectroscopy, gas-phase, aminoalcohol.

ABSTRACT Gas-phase vibrational spectra of 2-aminoethanol and 3-aminopropanol were recorded up to the third OH-stretching overtone using fourier transform infrared spectroscopy, cavity ringdown spectroscopy, and intracavity laser photoacoustic spectroscopy. The experimental investigation was supplemented by local mode calculations, and the intramolecular interactions were investigated using atoms in molecules (AIM) and non-covalent interactions (NCI) theory. All calculations were performed at the CCSD(T)-F12a/cc-pVDZ-F12 level of theory. For both compounds the most abundant conformer has a structure that allows for hydrogen bond interaction from the OH group to the N atom of the amino group (OH–N). The spectra show signals from both hydrogen bonded and free OH-stretches, implying the presence of several conformers. We observe hydrogen bond-like interactions in both compounds. The red-shift of the bonded OH-stretching frequency and intensity enhancement of the fundamental transition suggest that the hydrogen bond interaction is more pronounced in 3-aminopropanol. AIM analysis supports the presence of a hydrogen bond in 3-aminopropanol but not in 2aminoethanol, whereas NCI analysis shows hydrogen bonding in both compounds with the stronger interaction found in 3-aminopropanol.


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INTRODUCTION The studies of inter- and intra-molecular hydrogen bonded compounds in the Earth’s atmosphere have received increased attention as vibrational transitions in these hydrogen bonded species have been shown to be important in absorption of solar radiation and atmospheric photochemistry.1-5 One type of binary hydrogen bonded complex that has been thoroughly studied is the alcohol-amine complex.6-16 However, spectroscopic investigations of intermolecular hydrogen bonded complexes at atmospheric relevant conditions are difficult, due to the low abundances of these complexes in the gas phase.17, 18 The problem with low abundance can be circumvented by investigating compounds in which the hydrogen bonding can occur intramolecularly. The compounds used here as model systems for the investigation of hydrogen bonding between an OH-group and an NH2-group are 2-aminoethanol and 3aminopropanol. Intramolecular hydrogen bonding in 2-aminoethanol and 3-aminopropanol has been the subject of several experimental19-34 and computational investigations.35-45 The experimental techniques used in previous investigations include infrared and Raman spectroscopy,19-25 microwave spectroscopy,26, 27 photoelectron spectroscopy,28 penning ionization electron spectroscopy,29, 30 and electron diffraction.31 These studies agree that the conformation with hydrogen bond interaction from the OH group to the N atom of the amino group (OH–N) is the dominant structure in both 2-aminoethanol and 3-aminopropanol. Those are weak hydrogen bonds, for which the interaction energy is estimated to be less than 14 kJ/mol. 28, 35-37 For 2-aminoethanol the more stable conformer is the gauche conformer, and besides the hydrogen bond interaction the gauche effect is believed to play a major role in the conformational stability.46, 47


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Early infrared spectroscopic investigations of 2-aminoethanol and 3-aminopropanol in CCl4 solution, report the presence of several conformers based on the appearance of OH-stretches from hydrogen bonded and non-bonded conformers, respectively.19, 20 Matrix isolation infrared spectra of 2-aminoethanol and 3-aminopropanol also reveal the presence of several conformers, with the OH–N hydrogen bonded conformer being dominant.21, 22 Gas phase infrared spectra of the low-volatility aminoalcohols have been recorded at temperatures up to 180○C.22-24 Bands corresponding to both free and hydrogen bonded OH-stretching vibrations were observed and elevation of temperature led to an increase in signal from non-bonded conformers. Studies on a series of intramolecular interacting compounds [X(CH2)nOH, X = NH2, NR2, OH, OCH3, F, Cl, Br, I] in CCl4 revealed that the strongest hydrogen bonding interaction is found in aminoalcohols.20 This result is based on the frequency shift of the fundamental hydrogen bonded OH-stretching vibration and is explained by the high proton accepting property of the amino nitrogen.20 Both experimental21, 23 and computational investigations36 have shown that the intramolecular hydrogen bond interaction is stronger in 3-aminopropanol than in 2aminoethanol, since the longer carbon chain in the former compound makes it possible for the molecule to adopt a more favorable geometry for intramolecular interactions. Similar results have been obtained for the corresponding alkanediols.48-50 Infrared spectroscopy is a very useful method to investigate hydrogen bonding. It is well established that a hydrogen bond is associated with a red-shift of the XH-stretching frequency (X being a heavy atom such as O, N or S) together with an intensity enhancement of the fundamental transition.51 The magnitude of the red-shift increases with increasing hydrogen bond


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strength.52 The aim of this study is to describe the nature and strength of intramolecular hydrogen bonding in 2-aminoethanol and 3-aminopropanol. For both compounds the dominant conformer exhibits an OH–N hydrogen bond and consequently the OH-stretching spectra should provide the most direct information about the nature of this interaction. Comprehensive investigations have been performed on the fundamental transitions for both 2-aminoethanol and 3-aminopropanol,1925

but this is to our knowledge the first study of the overtone spectra for these molecules.

Previous studies on intramolecular hydrogen bonded compounds including diols,48, 49, 53 mercapto alcohols54 and dithiols54 have revealed the substantial advantages of overtone spectroscopy to address the question of hydrogen bond interaction strength and to resolve signals from different conformers otherwise unresolved in the fundamental region. EXPERIMENTAL METHODS The compounds 2-aminoethanol (Sigma-Aldrich, >99.5%) and 3-aminopropanol (SigmaAldrich, >99%) were dried with molecular sieves and degassed with several freeze-pump-thaw cycles before use. The vapor pressure is 0.2 and 0.3 mmHg for 2-aminoethanol and 3aminopropanol, respectively.55 Fourier transform infrared (FTIR) spectra of the fundamental and first overtone XH-stretch transition of 2-aminoethanol and 3-aminopropanol were recorded on a Bruker IFS 66v/S spectrometer. To observe the mid-infrared region from 1000-4000 cm-1, the instrument was set up with a combination of a globar source, a KBr beam splitter, and an MCT detector. For the near-infrared region from 6250-7500 cm-1, a tungsten light source, CaF2 beam splitter and an InGaAs detector were used. All experiments were performed with the instrument in the external mode, where the light source was directed through a CaF2 window into a 75 cm flow cell fitted


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with CaF2 windows and focused onto the detector with a parabolic mirror as previously described.56, 57 All spectra were recorded at 0.5 cm-1 resolution. The liquid sample was heated to 55○C to increase the vapor pressure, and the gaseous sample was entrained in a flow of N2 and introduced into the flow cell. The flow cell was kept at room temperature for all FTIR experiments. Cavity ringdown spectroscopy (CRD) was used to record the third (∆vXH = 4) XH-stretching overtones between 12500-13800 cm-1 with a resolution of 1 cm-1. The instrument is a pulsed CRD configuration whose setup has been explained in detail previously.57-59 To obtain the third overtone, mirrors centered at 755 nm and the LDS 759 laser dye was used. The liquid sample was heated to 55°C and the vapors were entrained in a flow of He and introduced into the CRD cavity. The CRD was operated with a mirror purge that had a flow rate of 0.5 SLPM (standard liters per minute) of He. The CRD sample inlet and cavity was wrapped in heating tape to maintain a temperature of 55 °C. Intracavity laser photoacoustic spectroscopy (ICL-PAS) was used to record spectra in the wavelengths regions 10150–10650 cm-1 and 12300–14000 cm-1 not accessible with the CRD setup. The ICL-PAS setup has been described in detail elsewhere60, 61 with the only difference in the present experiment being the use of a solid-state (Coherent, Verdi 12W) pump laser. The ICL-PAS spectra were recorded with a tunable titanium:sapphire laser (Coherent 890) with two different wave output couplers, covering the region from 10150 to 10650 cm-1 and the region from 12300 to 14000 cm-1, respectively. The resolution of the laser is ~1 cm-1. The sample was transferred to the photoacoustic cell through a vacuum line, with the use of a liquid nitrogen cool trap on the cell sample arm. To enhance the photoacoustic signal, ca 200 Torr of argon buffer gas was added to the photoacoustic cell.62 All experiments were performed at room temperature.


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COMPUTATIONAL METHODS A systematic conformer search was performed using the B3LYP functional with the 6-31+G(d) basis set.63 The five lowest energy conformers of 2-aminoethanol and four lowest energy conformers of 3-aminopropanol were reoptimized and harmonic frequencies were calculated using the B3LYP functional with the aug-cc-pVTZ basis set. Finally, a geometry optimization was performed with the explicitly correlated coupled cluster singles, doubles and perturbative triples method [CCSD(T)-F12].64, 65 The CCSD(T)-F12a variant, which is recommended for double-ζ orbital basis sets was used.66, 67 The cc-pVDZ-F12 orbital basis set used in the CCSD(T)-F12 calculations has been specifically optimized for use with explicitly correlated methods. Boltzmann populations were determined from the CCSD(T)F12a/cc-pVDZ-F12 energy using the B3LYP/aug-cc-pVTZ thermal correction. All local mode parameters were obtained at the CCSD(T)-F12a/cc-pVDZ-F12 level of theory.68 All DFT calculations were carried out using Gaussian 09 programs69 and the CCSD(T)-F12a calculations were done using Molpro 2010.1.70 For the CCSD(T)-F12a calculations optimization threshold criteria were set to step = 1 × 10-6 au, grad = 1 × 10-6 au, and energy = 1 × 10-8 au. The threshold for single point energy calculation were set to energy = 1 × 10-9 au. To minimize basis set incompleteness error the CCSD(T)-F12 geminal exponent was set to 0.9, which is the optimal value for the cc-pVDZ-F12 basis set.71 The CCSD(T)-F12a dipole moment was calculated from a finite field approach with a field strength of 0.0001 au All electronic structure calculations assumed a frozen core (C, 1s; N, 1s; O 1s). Visualizations of molecular structures were carried out using the GaussView 4.1 program.72


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Intramolecular hydrogen bonding interactions were investigated using the Atoms in Molecules (AIM) and Non-Covalent Interactions (NCI) theories with the AIMALL and NCIPLOT programs, respectively.73, 74 The necessary wave function files for both AIMALL and NCIPLOT were generated using MOLPRO with the CCSD(T)-F12a/cc-pVDZ-F12 method. VIBRATIONAL THEORY The vibrations of XH-stretching oscillators, where X is a heavy atom (in this study X = N, O), are anharmonic and their vibrational overtone spectra are generally best described with the use of a local mode model.75-78 Isolated anharmonic oscillator, OH-stretching mode. The OH-stretching mode can be described as an isolated anharmonic oscillator, assuming it has only minimal coupling to other modes. The OH-stretching modes are calculated with the use of the anharmonic local mode model and the OH bond described by a Morse oscillator.79 The vibrational transition energies are given by: / ,

(1)

where Ev is the energy of the vibrationally excited state, E0 is the energy of the vibrational ground state, h is Planck’s constant, c is the speed of light, is the vibrational quantum number,

is the local mode harmonic frequency, and is the anharmonicity. This leads to the twoparameter Morse oscillator energy expression: / 1 ,


(2)

8

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where is the transition energy in cm-1 from =0 to . The and terms are obtained from the second (fii), third (fiii) and fourth (fiv) order derivatives of the potential energy surface with respect to the oscillator stretching coordinate:49

!

(3)

,

"# $ ,

(4)

where Gii is the reciprocal of the reduced mass of the oscillator. The derivatives of the potential energy surface are obtained by fitting an eighth order polynomial to a nine point potential energy grid calculated in the range from -0.20 to 0.20 Å around the equilibrium bond distance in steps of 0.05 Å. The dimensionless oscillator strength, f, of a vibrational transition from the ground state, g, to the excited state, e, is given by:80, 81

" 4.702 * 10+, - . /+ 012 34512 3 ,

(5)

where 12 is the transition energy in cm-1 and 4512 is the transition dipole moment matrix element over the excited state and ground state vibrational wavefunction, 4512 671 345372 8, in D. The dipole moment function is approximated by a Taylor series expansion in the internal displacement coordinate, q, around equilibrium: 459 ∑# 45# 9 # ,


(6)

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where the coefficients 45# are given by: ?5 ; = >

45# #! =@ A @B .

(7)

The coefficients are obtained from a calculated one dimensional dipole moment grid. The grid points are calculated in the range from -0.20 to 0.20 Å around the equilibrium bond distance in steps of 0.05 Å for a total of nine points. The series expansion of the dipole moment function is limited to sixth order. Harmonically coupled anharmonic oscillator: NH-stretching modes. In order to describe the NH-stretches it is necessary to include coupling between the two NH-oscillators in the NH2group.82 This is done by the harmonically coupled anharmonic oscillator (HCAO) local mode model which has been described in detail elsewhere.81, 83 Since the two NH-oscillators experience slightly different environments, they will have different and . The vibrational transition energies without coupling can be approximated by: C / ; ; ; ; ; ,

(8)

where the subscripts refer to each NH-stretching oscillator. The total vibrational manifold is and are calculated as described for the isolated anharmonic oscillator. ; , and The NH-stretching vibrational states are labeled | ; 〉FG; | 〉FG , in which ; and denote the vibrational state of each NH-stretching oscillator. The perturbation to the vibrational transition energy including the intramanifold harmonic coupling is:81, 83 C′/ I′J;K J J; JK ,


(9)

10

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where JK and J are the step-up and step-down operators known from harmonic oscillators. The effective coupling parameter contains both kinetic and potential coupling, which for two nonidentical oscillators becomes:81 I L M

NO P

Q

QR

R KQS

;

$

T

TT

U ,

(10)

where θ is the HNH angle, mH and mN are the atomic masses and f11, f22 and f12 are the second order derivatives of a two-dimensional potential energy surface. The oscillator strength for the coupled NH-oscillators is calculated as described for the isolated anharmonic oscillator (eq. 5). The dipole moment function now includes both diagonal and mixed terms:83 W

459; , 9 ∑#,W 45#,W 9;# 9

(11)

The coefficients are obtained from a two-dimensional dipole moment grid. We limit the expansion to sixth order for the diagonal terms and third order for the mixed terms.60 Both twodimensional dipole moment grid and potential energy surface are calculated in the range from 0.20 to 0.20 Å around the equilibrium bond distance in steps of 0.05 Å for a total of 81 points.


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RESULTS AND DISCUSSION Geometries and conformer distribution. The five lowest energy conformers of 2-aminoethanol and four lowest energy conformers of 3-aminopropanol are shown in Figure 1. The calculated lowest energy structures, 2A and 3A, are in accordance with previous investigations.36, 38, 40, 46, 47 Relative energies and abundances are presented in Table 1. The relative abundances are the calculated Boltzmann distribution at room temperature. 3-aminopropanol is dominated by a single conformer, 3A (85%) at room temperature, whereas 2-aminoethanol has a slightly broader conformer distribution with the most abundant conformer, 2A, constituting 73%. The structural degeneracy is 2 for all conformers and does not change the abundances. Selected optimized geometric parameters for 2A-E and 3A-D are given in Table 2. For the conformers where an OH–N hydrogen bond interaction is structurally permitted, the hydrogen bond length (rOH–N) and the OH–N angle (∠OH–N) are presented in Table 2. It is evident that the

Figure 1. Geometries of the five lowest energy conformers of 2-aminoethanol (upper) and four lowest energy conformers of 3-aminopropanol (lower) optimized at CCSD(T)-F12a/cc-pVDZF12 level of theory.


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OH bond distance (rOH) is slightly longer for 3A than for 2A (~0.003 Å). The distance between H and N (rOH–N) decreases from 2A to 3A and simultaneously, the OH–N angle (∠OH–N) increases and the hydrogen bond becomes more linear. These three changes indicate a greater hydrogen bond interaction in 3-aminopropanol than in 2-aminoethanol. The second most abundant conformer of 3-aminopropanol (3B) has a structure that also seems to allow for hydrogen bond interactions from the OH-group to the N of the amino group. The geometrical parameters point to a weaker interaction: shorter rOH, longer rOH–N and more bent ∠OH–N than in 3A. We expect to see a strong signal in the fundamental region from the hydrogen bonded OHstretch (OHb) of the most abundant conformers 2A and 3A. Although the conformers 2B-D and 3C-D with free OH-stretches (OHf) have low abundances we also expect to observe the OHfstretching fundamental and overtone transitions from these conformers. Previous studies on mercapto-ethanol,54 which has similar abundances for OHf-conformers, clearly showed overtone spectra in which the OHf-transitions could be identified. In the following, we assign OHb-, OHf-, and NH-stretching transitions in the experimental spectra.


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Table 1. Relative energies (kJ/mol) and abundances of the five lowest energy conformers of 2-aminoethanol and four lowest energy conformers of 3-aminopropanol. ∆Ea

∆Ezpvea ∆Gb

F(%)c

F(%)c

(298K)

(298K)

(328K)

2A

0.0

0.0

0.0

73.3

72.1

2B

7.0

5.7

5.0

9.8

10.2

2C

8.0

6.4

5.6

7.6

8.0

2D

8.8

7.3

6.6

5.1

5.3

2E

8.7

7.5

7.1

4.2

4.4

3A

0.0

0.0

0.0

85.2

83.7

3B

8.1

7.5

6.2

6.9

7.3

3C

11.6

9.7

8.5

2.7

2.9

3D

12.6

9.4

6.9

5.2

6.1

a

Relative electronic energies (∆E) and energies including zero-point vibrational energy (∆Ezpve) calculated from CCSD(T)-F12a/cc-VDZ-F12 electronic energies with ZPVE obtained using B3LYP/aug-cc-pVTZ harmonic frequencies. b Gibbs Free Energy (∆G) calculated from CCSD(T)-F12a/cc-VDZ-F12 electronic energies with ZPVE, enthalpy, and entropy contributions calculated using B3LYP/aug-cc-pVTZ.. c Relative abundances at 298 and 328 K, respectively, calculated with the ∆G obtained at the corresponding temperature.


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Table 2. Structural parameters of the five lowest energy conformers of 2-aminoethanol and four lowest energy conformers of 3-aminopropanol.a rOH

rNH1

rNH2

rOH–N

∠OH–N

2A

0.9656

1.0132

1.0111

2.2366

115.55

2B

0.9586

1.0122

1.0135

-

-

2C

0.9587

1.0139

1.0136

-

-

2D

0.9604

1.0138

1.0142

-

-

2E

0.9603

1.0136

1.0118

-

-

3A

0.9688

1.0126

1.0143

2.0032

141.85

3B

0.9644

1.0142

1.0128

2.3190

125.80

3C

0.9611

1.0125

1.0130

-

-

3D

0.9588

1.0137

1.0122

-

-

a

Bond lengths in Å and angles in degrees. Calculated at the CCSD(T)-F12a/cc-VDZ-F12 level of theory.


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Table 3. Observed and calculated OHb-stretching transitions for 2-aminoethanol experimentala

2Ab

∆v

vexp

FWHM

Rel I

vcalc

Rel f

1

3570

37

1.0

3568

1.0

2

6944

56

3 × 10-2

6960

3.0 × 10-2

3

-

-

-

10175

1.4 × 10-3

4

-

-

-

13213

9.9 × 10-5

5

-

-

-

16074

9.3 × 10-6

Experimental wavenumber ( and Full Width Half Maximum (FWHM) in cm-1. Relative intensities (Rel I) with the fundamental OHb-stretching transition set to 1.0. b Calculated local mode wavenumber ( in cm-1 and Boltzmann weighted relative oscillator strength (Rel f). Oscillator strengths are relative to the fundamental OHb-stretching transition which has a calculated oscillator strength of 1.1 × 10-5. All calculations are at the CCSD(T)-F12a/cc-pVDZF12 level of theory. a


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Table 4. Observed and calculated OHf-stretching transitions for 2-aminoethanol. experimentala ∆v

vexp

FWHM

1

3679

57

2

7130

61

7199

57

2Bb Rel I 0.24c 0.05

3

4

5

c

2Cb

2Db

2Eb

vcalc

Rel f

vcalc

Rel f

vcalc

Rel f

vcalc

Rel f

3695

4.9 × 10-2

3693

3.8 × 10-2

3666

2.0 × 10-2

3666

1.5 × 10-2

-

-

-

-

7158

3.3 × 10-3

7157

2.6 × 10-3

7217

7.8 × 10-3

7213

6.1 × 10-3

-

-

-

-

10475

1.1 × 10-4

-

-

13618

5.8 × 10-6

10450

63

-

-

-

-

-

10476

1.3 × 10

10543

50

-

10567

3.1 × 10-4

10561

2.4 × 10-4

-

-

-4

-6

13586

63

-

-

-

-

-

13620

7.3 × 10

13718

46

-

13745

1.5 × 10-5

13737

1.2 × 10-5

-

-

-

-

-

-

-

-

-

-

-

16591

5.6 × 10-7

16588

4.9 × 10-7

-

-

-

16750

1.2 × 10-6

16740

8.7 × 10-7

-

-

-

-

Experimental wavenumber ( and Full Width Half Maximum (FWHM) in cm-1. Relative intensities (Rel I) with the fundamental OHb-stretching transition set to 1.0. b Calculated local mode wavenumber ( in cm-1 and Boltzmann weighted relative oscillator strength (Rel f). Oscillator strengths are relative to the fundamental OHb-stretching transition of conformer 2A which has a calculated oscillator strength of 1.1 × 10-5. All calculations are at the CCSD(T)-F12a/cc-pVDZ-F12 level of theory. c The experimental OHfstretching transitions cannot be resolved and the intensity is the sum of all OHf-stretching bands. a


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Table 5. Observed and calculated OHb- stretching transitions for 3-aminopropanol. experimentala

3Ab

3Bb

∆v

νexp

FWHM

Rel I

νcalc

Rel f

νcalc

Rel f

1

3485

30

1.0

3457

1.0

-

-

3589

26

0.02

-

-

3600

1.8 × 10-2

2

6596c

87c

0.2c

6710

3.2 × 10-3

7024

4.6 × 10-4

3

-

-

-

9761

2.8 × 10-4

10272

2.2 × 10-5

4

-

-

-

12609

2.6 × 10-5

13343

1.4 × 10-6

5

-

-

-

15254

3.4 × 10-6

16238

1.6 × 10-7

Experimental wavenumber ( and Full Width Half Maximum (FWHM) in cm-1. Relative intensities (Rel I) with the fundamental OHb-stretching transition set to 1.0. b Calculated local mode wavenumber ( in cm-1 and Boltzmann weighted relative oscillator strength (Rel f). Oscillator strengths are relative to the fundamental OHb-stretching transition of conformer 3A which has a calculated oscillator strength of 5.1 × 10-5. All calculations are at the CCSD(T)F12a/cc-pVDZ-F12 level of theory. c Mainly assigned to NH-stretch but also contains OHb, wavenumber is position only an estimate. a


18

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Table 6. Observed and calculated OHf-stretching transitions for 3-aminopropanol. experimentala ∆v

vexp

FWHM

1

3675

44

2

7132

32

3Cb Rel I c

0.05

c

3Db

vcalc

Rel f

vcalc

Rel f

3657

2.1 × 10-3

3691

4.1 × 10-3

7141

2.7 × 10-4

-

-

-

-

7209

7.6 × 10-4

0.01 3

4

5

7180

43

10452

46

-

10452

1.1 × 10-5

-

-

10525

52

-

-

-

10555

3.1 × 10-5

13595

40

-

13591

5.8 × 10-7

-

-

13682

45

-

-

-

13727

1.5 × 10-6

-

-

-

16556

4.9 × 10-8

16727

1.2 × 10-7

Experimental wavenumber ( and Full Width Half Maximum (FWHM) in cm-1. Relative intensities (Rel I) with the fundamental OHb-stretching transition set to 1.0. b Calculated local mode wavenumber ( in cm-1 and Boltzmann weighted relative oscillator strength (Rel f). Oscillator strengths are relative to the fundamental OHb-stretching transition of conformer 3A which has a calculated oscillator strength of 5.1 × 10-5. All calculations are at the CCSD(T)F12a/cc-pVDZ-F12 level of theory. c The experimental OHf-stretching transitions cannot be resolved and the intensity is the sum of all OHf-stretching bands. a


19


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Page 20 of 53

Table 7. Observed and calculated NH-stretching transitions for 2-aminoethanol. experimentala state

vexp

FWHM

|1〉|0〉

3346

27

3414

48

|0〉|1〉 |2〉|0〉 |0〉|2〉 |3〉|0〉 |0〉|3〉 |4〉|0〉 |0〉|4〉 |3〉|1〉 |1〉|3〉

|5〉|0〉d |0〉|5〉d

6613

115

2Ab Rel I 1.0c

0.5c

v calc

Rel f

3369

0.22

3456

0.78

6627

0.34

6691

0.54

-

-

-

9735

1.8 × 10-2

-

-

-

9821

2.2 × 10-2

-

-

-

12690

1.0 × 10-3

12668

83

-

12810

1.1 × 10-3

12998

77

-

13128

3.8 × 10-5

13071

76

-

13228

3.1 × 10-4

15303

168

-

15498

8.5 × 10-5

15464

152

-

15651

8.8 × 10-5

Experimental wavenumber ( and Full Width Half Maximum (FWHM) in cm-1. Relative intensities (Rel I) with the sum of the fundamental NH-stretching transitions set to 1.0. b Calculated local mode wavenumber ( in cm-1 and Boltzmann weighted relative oscillator strength (Rel f). Oscillator strengths are relative to the sum of the two fundamental NHstretching transitions of conformer 2A which has a calculated oscillator strength of 4.8 × 10-7. All calculations are at the CCSD(T)-F12a/cc-pVDZ-F12 level of theory. c The two experimental NH-stretching transitions cannot be resolved and the intensity is the sum of both NH-stretching bands. The band also contains the OHb-stretching trantision d CRD spectra of the NH-stretching transitions in the fourth overtone region are presented in the Supporting Information. a


20

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Table 8. Observed and calculated NH-stretching transitions for 3-aminopropanol. experimentala state

vexp

FWHM

|1〉|0〉

3288

54

3374

42

6596

87

|0〉|1〉 |2〉|0〉 |0〉|2〉 |3〉|0〉 |0〉|3〉 |4〉|0〉 |0〉|4〉 |1〉|3〉

|5〉|0〉d |0〉|5〉d

3Ab Rel I 1.0c

0.2c

vcalc

Rel f

3354

0.21

3438

0.79

6600

0.34

6653

0.59

-

-

-

9697

1.7 × 10-2

-

-

-

9759

2.6 × 10-2

-

-

-

12641

9.6 × 10-4

12587

71

-

12725

1.4 × 10-3

12999

81

-

13158

3.5 × 10-4

15221

104

-

15436

8.1 × 10-5

15361

85

-

15542

1.0 × 10-4

Experimental wavenumber ( and Full Width Half Maximum (FWHM) in cm-1. Relative intensities (Rel I) with the fundamental NH-stretching transitions set to 1.0. b Calculated local mode wavenumber ( in cm-1 and Boltzmann weighted relative oscillator strength (Rel f). Oscillator strengths are relative to the sum of the two fundamental NH-stretching transitions of conformer 3A which has a calculated oscillator strength of 4.6 × 10-7. All calculations are at the CCSD(T)-F12a/cc-pVDZ-F12 level of theory. c The two experimental NH-stretching transitions cannot be resolved and the intensity is the sum of both NH-stretching bands. d CRD spectra of the NH-stretching transitions in the fourth overtone region are presented in the Supporting Information. a


21


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Page 22 of 53

Observed and Calculated Fundamental Spectra. Figure 2 shows the fundamental IR spectra of 2-aminoethanol and 3-aminopropanol. Experimental peak positions, full width at half maximum (FWHM) and relative intensities for signals assigned to OH-stretching transitions are presented in Tables 3–6 together with calculated local mode anharmonic transitions. We have set the intensity of the fundamental OHb-stretching transition to 1.0 as a reference point for reporting the relative intensities. This corresponds to calculated oscillator strengths of the anharmonic OHb-stretching transition of 1.1 × 10-5 and 5.1 × 10-5 for 2-aminoethanol and 3-aminopropanol, respectively. Experimental NH-stretching transitions are presented in Tables 7 and 8 together with the HCAO calculated local mode transitions for the most abundant conformers, 2A and 3A respectively. The signal corresponding to each NH-oscillator cannot be resolved in the fundamental region. The intensity of the sum of the fundamental NH-stretching transitions is set to 1.0 as a reference point for relative intensities. This corresponds to calculated oscillator

Figure 2. The vapor-phase fundamental spectra of 2-aminoethanol (top) and 3aminopropanol (bottom).


22

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strengths of the NH-stretching transitions of 4.8 × 10-7 for 2-aminoethanol and 4.6 × 10-7 for 3aminopropanol. These fundamental NH-stretching transitions are very weak compared to most other fundamental transitions. Calculated harmonic and local mode anharmonic OH- and NHstretching transitions for all conformers are presented in the Supporting Information. Absorption features assigned to CH-stretches (

Intramolecular Interactions in 2-Aminoethanol and 3-Aminopropanol

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