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Overtone Spectra of 2-Mercaptoethanol and 1,2-Ethanedithiol Benjamin J. Miller, Mivsam Yekutiel, A. Helena Sodergren, and Daryl L. Howard† Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand
Meghan E. Dunn and Veronica Vaida Department of Chemistry and Biochemistry and CIRES, UniVersity of Colorado, Campus Box 215, Boulder, Colorado 80309, United States
Henrik G. Kjaergaard* Department of Chemistry, UniVersity of Copenhagen, UniVersitetsparken 5, Copenhagen Ø, Denmark ReceiVed: NoVember 27, 2009; ReVised Manuscript ReceiVed: October 21, 2010
Vibrational spectra of vapor-phase 1,2-ethanedithiol and 2-mercaptoethanol were recorded to investigate weak intramolecular interactions. The spectra were recorded with conventional absorption spectroscopy and laser photoacoustic spectroscopy in the 2000-11 000 cm-1 region. The room temperature spectra of each molecule are complicated by contributions from several conformers. Anharmonic oscillator local-mode calculations of the OH- and SH-stretching transitions have been performed to facilitate assignment of the different conformers in the spectra. We observe evidence of hydrogen-bond-like interactions from OH to S, but not from SH to O or S. The OH to S intramolecular interaction in 2-mercaptoethanol is weak and comparable to that found in the OH to O interaction in ethylene glycol. Introduction Intramolecular interactions are investigated using the simple model systems of 1,2-ethanedithiol (EDT) and 2-mercaptoethanol (2-thioethan-1-ol, 2ME) in comparison with ethylene glycol (EG). EDT is the simplest molecule with two vicinal thiol groups and 2ME is the simplest molecule with a thiol group vicinal to a hydroxyl group. Both EDT and 2ME can serve as simple models for more complex systems, such as those found in biological molecules, where thiol groups can play an important role.1 The close proximity of the OH and SH substituents in EDT and 2ME allow the possibility of weak intramolecular interactions, in particular, to or from a thiol group. 2ME provides an example where it is possible to directly investigate the competitive interaction between OH and S, or SH and O. EG is perhaps the most studied molecule with vicinal groups.2-4 In EG, the two OH groups are positioned with an OH · · · O angle far from the linear conformation optimal for hydrogen bonding. EG and other vicinal diols lack a (3, -1) bond critical point, which is often stated as a requirement for a bond.3,4 In EG, there is evidence for hydrogen-bond-like interactions as indicated in red shifting of the associated OHstretching vibrations.2 Accordingly, there has been much discussion in the literature of the existence of a hydrogen bond in vicinal diols.2-4 The intramolecular hydrogen-bond-like interaction between vicinal OH groups is expected to be weak.2,5 However, if the carbon chain between the OH groups is increased, as for example in propan-1,3-diol, the two OH groups can align more favorably and the interaction increases such that * Corresponding author. E-mail:
[email protected]. Fax: 45-35320322. Phone: 45-35320334. † Present address: Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia.
a hydrogen bond can be identified both spectroscopically and via a bond critical point.5,6 Infrared spectroscopy is one of the classical methods for identifying hydrogen bonds. It is well-suited because it gives direct information about the groups of the molecule that participate in the formation of the hydrogen bonds.7 A red-shift in the XH-stretching frequency (where X is C, N, O, S, ...) and an increase in intensity in the fundamental stretching transition of the XH bond donating the H-atom to the hydrogen bond are generally considered spectroscopic signatures for a hydrogen bond. These changes in the fundamental region are followed by a larger red-shift and a decrease in intensity in the first XHstretching overtone.5,8-12 These spectroscopic signatures of the fundamental and first overtone XH-stretching vibration, which are typical of hydrogen bonding, have been observed in a number of molecules and complexes.5,9,10,13,14 The magnitude of these red-shifts and intensity changes increase with increasing hydrogen-bond strength.5,14-16 Where the interaction is very weak and the existence of hydrogen bonding is questionable, these weak interactions can be termed hydrogen-bond-like interactions. For vicinal diols the fundamental frequency shift is about 30-50 cm-1, and the estimated interaction strength is approximately between 100 and 200 cm-1; however, there is as mentioned a lack of a bond critical point.17,18 In a weakly bound complex, such as H2O-N2, which has a hydrogen-bond energy of approximately 450 cm-1, the spectroscopic changes are even smaller; however, a bond critical point is found.19 It is sometimes difficult to distinguish the red-shift and intensity variation in the XH-stretching mode of weakly hydrogen bonded XH substituents in the fundamental, as the band for the hydrogen-bonded conformer can overlap with bands from free XH substituents, especially if several conformers are present. In that case, it is convenient to probe the higher vibrational overtones for evidence of weak interactions. Small variations
10.1021/jp9112798 2010 American Chemical Society Published on Web 11/10/2010
Overtone Spectra of 2ME and EDT in frequency and anharmonicity become more pronounced as higher vibrational overtone transitions become excited.16,20,21 There is strong interest in intermolecular hydrogen bonding in complexes.22-24 However, room-temperature studies of these weakly bound complexes are complicated by their low equilibrium constants and correspondingly short lifetimes.22,25-29 To counteract this disadvantage, we have in the past investigated intramolecular hydrogen-bond-like interactions by probing OHstretching overtones.2,5,8,14,17,30-36 In the present study, we use room-temperature, vapor-phase OH- and SH-stretching vibrational spectroscopy supported by DFT and ab initio calculations of local mode XH-stretching transitions, abundance of conformers, and bond critical points to investigate the intramolecular interactions in 2ME and EDT. Experimental studies of hydrogen bonding involving sulfur are rare.17,37-58 The majority of these studies concern intermolecular hydrogen bonding between H2S and other small molecules.37,42,43,51,52 The lack of information is no doubt due to the comparative weakness of the interaction and the lack of intensity from SH-stretching transitions in general.59 Before the 1960s, there was doubt as to whether thiol groups even participated in hydrogen bonding at all.7,39 The bulk of this previous work deals with matrix-isolated and liquid-phase spectroscopy and not vapor-phase spectroscopy.40-51 Recently, there has been an experimental vapor-phase study involving hydrogen bonding from OH to S.17 Howard and Kjaergaard used variable-temperature FTIR and ab initio calculations to find that the hydrogen bond between methanol and dimethyl sulfide (DMS) was similar in strength to the hydrogen bond between methanol and dimethyl ether (DME).17 There have also been jet-cooled experimental spectra recorded that compare OH · · · S and OH · · · O intermolecular hydrogen bonding.52-55 Biswal et al. used jet-cooled techniques and B3LYP and MP2 calculations to ascertain that the hydroxyl groups on phenol, p-cresol, and 2-napthol hydrogen bond to DMS as effectively as they hydrogen bond to H2O.54 Biswal et al. also used jet-cooled techniques to study complexes of p-cresol with H2S or H2O. They find that the hydrogen bond in the p-cresol-H2S complex is about half as strong as the one found in p-cresol-H2O.55 In 2010, Biswal et al. also studied the complexes p-cresol-diethyl sulfide, p-cresol-methanethiol, and p-cresol-ethanethiol.52,53 These studies show that S is a weaker hydrogen-bond acceptor than O, even if the OH-red shift in p-cresol is larger when interacting with an S atom, compared with an O atom acceptor, in agreement with the earlier results for methanol and DMS and DME by Howard and Kjaergaard.17 Experimental Section 2-Mercaptoethanol (Sigma-Aldrich, g99%) and 1,2-ethanedithiol (Fluka, g98%) were dried with molecular sieves and degassed with multiple freeze-pump-thaw cycles performed on a vacuum line before use. Experimental work was carried out at the University of Colorado, Boulder, the University of Otago, Dunedin, and the University of Copenhagen. At Boulder, the vapor-phase spectra were recorded on a Bruker IFS 66v/S FTIR at 1 cm-1 resolution. The spectra were recorded with a gas cell having CaF2 windows and a path length of 15 or 71.4 cm. Vapor pressures were measured using a pressure gauge while the cell was filled with sample. Spectra were recorded in the mid-infrared from 1000 to 8000 cm-1, with a Globar source, KBr beamsplitter, and MCT detector, and in the near-infrared from 2500 to 11 700 cm-1, with a tungsten source, CaF2 beamsplitter, and InSb detector. Spectra in the IR region of 1000-7500 cm-1 were recorded at Otago either with a 2.4 m or a 4.8 m path length, multipass
J. Phys. Chem. A, Vol. 114, No. 48, 2010 12693 gas cell (both from Infrared Analysis Inc.) on a PerkinElmer Spectrum 100 FT-IR spectrometer with a quartz/halogen source, a KBr beamsplitter, and a FR-DTGS detector. The 2.4 m cell was equipped with a heating jacket and the temperature was controlled by a Digi-Sense electronic temperature controller (Eutech Instruments Pte. Ltd. Model 68900-03). The spectra in the NIR region of 5500-9900 cm-1 were recorded with a variable path length Wilks gas cell, set at 14.25 m, on a Varian Cary 500 spectrophotometer. The Cary spectra were recorded with the spectrometer in double-beam mode with the Wilks cell in the front beam and a ND filter (with an optical density of 0.12) placed in the reference beam path. All vapor pressures for the different experiments were monitored using a high vacuum diaphragm manometer (Varian DV100) coupled to a pressure gauge (Varian model 6543-25-039). Also at Otago, room temperature vapor-phase spectra in the ∆VCH ) 4 region of EDT were recorded with intracavity laser photoacoustic spectroscopy (ICL-PAS). The ICL-PAS setup has been described in detail elsewhere.60-62 The ICL-PAS spectra were recorded with a tunable titanium:sapphire laser (Coherent 890) with both the long-wave and short-wave output couplers which covered the region from approximately 10 500 to 13 800 cm-1. The line width of the titanium:sapphire laser is approximately 1 cm-1. In our photoacoustic cell, we use a Knowles EK3133 microphone to detect the photoacoustic signal. Because the photoacoustic signal is proportional to absorbance with an unknown constant, we do not obtain absolute absorbance data from the photoacoustic spectra. In Copenhagen, the room-temperature vapor-phase spectra in the ∆VOH ) 3 region of 2ME was recorded with ICL-PAS. The ICL-PAS setup in Copenhagen is similar to the one at Otago with the main difference being a change to a solid-state (Coherent, Verdi 12W) pump laser. Theory and Calculations. The dimensionless oscillator strength f of a vibrational transition within the same electronic state from the vibrational ground state g to an excited vibrational state e can be written as63
feg )
4πme 3e2p
f |2 νeg |µ eg
(1)
where νeg is the frequency of the transition and the transition b|Ψg〉 is the overlap dipole moment matrix element b µeg ) 〈Ψe|µ between the wave function of the ground state Ψg, the wave function of the excited state Ψe, and the dipole moment function b µ. The other parameters are the usual physical constants. The dimensionless oscillator strength can be expressed in units commonly used in spectroscopy, and eq 1 becomes64
f |2 feg ) 4.701 65 × 10-7ν˜ eg |µ eg
(2)
where b µeg is expressed in debye (D) and ν˜ in wavenumbers (cm-1). Thus, in order to calculate vibrational transition intensities, we need to calculate the wave functions and energies of the vibrational states and also the dipole moment as a function of the internal XH-stretching coordinate, where X is any heavy atom (O or S). The wave functions and energies are obtained by solving the vibrational Schro¨dinger equation, which has been solved within an anharmonic oscillator (AO) local mode approximation with the XH bond described by a Morse oscillator.20,65,66 The vibrational transition energy levels are given by
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Miller et al.
(3)
where ν is the vibrational quantum number. The Morse oscillator harmonic wavenumber ω ˜ and anharmonicity parameter ω ˜ x can be found from2
ω ˜ )
ω ˜x )
(
giifii 2πc
pgii 5fiii2 fiv 2πcfii 48fii 16
(4)
)
(5)
where fii, fiii, and fiv are the second-, third-, and fourth-order force constants of the potential energy curve along the XH bond. The force constants were obtained as energy derivatives by fitting an eighth-order polynomial to nine potential energy points calculated in the range from -0.20 to 0.20 Å around equilibrium with a stepsize of 0.05 Å. The dipole moment function in eq 2 is expanded as a Taylor series around equilibrium. The dipole moment functions have been calculated in the range from -0.30 to 0.40 Å around equilibrium with a stepsize of 0.05 Å and are fitted to a sixthorder polynomial.67 The overlaps between the Morse oscillator functions and the displacement coordinate are evaluated analytically. The analytical Morse overlap matrices have been given elsewhere.68,69 Initially, a systematic conformer search was completed with SPARTAN using the Becke three-parameter-Lee-Yang-Parr (B3LYP) hybrid functional with a 6-31+G(d) basis set.70 The five lowest energy conformers of each species studied in this work were further optimized with the B3LYP, M06-2x, and MP2 methods and the Dunning type triple-ζ correlation consistent basis set augmented with diffuse basis functions [augcc-pV(T+d)Z] and tight d basis functions added to the sulfur atoms. The tight d basis functions have been shown to improve the energies of second-row atoms.71-75 Single point energies were calculated with the CCSD(T)/aug-cc-pVTZ and CCSD(T)/ cc-pVTZ methods on the B3LYP- and MP2-optimized structures. In addition, we have used the QB3-CBS composite method on the five lowest energy conformers of each species. Free energies were calculated using the CBS-QB3 method, implemented in G09, with the MP2-optimized structure as the initial structure for each of the conformers. Where not specified, calculations were performed using MOLPRO 2006.1 with the optimization threshold criteria set to step ) 1 × 10-5 au, grid ) 1 × 10-5 au, and energy ) 1 × 10-7 au.76 Single point energy criteria were set to orbital )1 × 10-8 au and energy ) 1 × 10-9 au. For B3LYP calculations, only a single point energy criterion was set to grid ) 1 × 10-8 au. The atoms in molecules calculations were done with Gaussian 09 with the M06-2x/aug-cc-pV(T+d)Z-optimized structures and densities.77 All five 2ME conformers investigated have a structural degeneracy of 4. Therefore, structural degeneracy does not play a part in the room-temperature Boltzmann populations. EDT conformer 2 has Ci symmetry and conformers 1 and 3 have C2 symmetry and all three have a structural degeneracy of 2, while conformers 4 and 5 have a structural degeneracy of 4. This lowers the energy of conformers 4 and 5 by 144 cm-1 and raises their contribution to the room-temperature Boltzmann population accordingly.
Results and Discussion In Tables 1 and 2 we present selected dihedral angles and relative energies and abundances for the five lowest energy MP2/ aug-cc-pV(T+d)Z-optimized structures of 2ME and EDT. The corresponding structures are shown in Figure 1. The MP2/augcc-pV(T+d)Z-optimized structures are very similar to the B3LYP/aug-cc-pV(T+d)Z-optimized structures. Our calculated structural parameters are in good agreement with those from structures determined by microwave spectroscopy and other ab initio calculations.78-81 The MP2/aug-cc-pV(T+d)Z and B3LYP/ aug-cc-pV(T+d)Z calculated harmonic frequencies are given in Tables S1 and S2 for 2ME and Tables S3 and S4 for EDT (Supporting Information) and have no imaginary frequencies. The structures shown in Figure 1 are arranged with the lowest energy conformer, from our initial B3LYP/6-31+G(d) conformer search, at the top. The relative energies of the conformers are given in Tables 1 and 2 for a few of the methods used. The variation in relative energies between the MP2/aug-cc-pV(T+d)Z and CCSD(T)/cc-pVTZ//MP2/aug-cc-pV(T+d)Z and CBS-QB3 calculations is within 100 cm-1, in line with what is expected. The free energies are somewhat different and do effect the relative abundances. The abundances based on the free energies and corrected for structural degeneracies are given in Tables 1 and 2. For 2ME, we find conformer 1 and 2 are the most abundant conformers, with room-temperature Boltzmann populations of approximately 40% each. The structural arrangement of conformer 1 is such that it could facilitate an OH · · · S intramolecular hydrogen-bond-like interaction. Conformer 2 of 2ME has a structure that we believe would not participate in any intramolecular hydrogen-bond-like interactions, as the OH and SH groups are not oriented toward any lone pair electron density. Conformers 3 and 4 of 2ME may be able to form SH · · · O intramolecular hydrogen-bond-like interactions, and the structure of conformer 5 has no possibility of forming any intramolecular hydrogen-bond-like interactions. For EDT, the lowest energy conformer is sensitive to the choice of ab initio method. The B3LYP/aug-cc-pV(T+d)Zcalculated energies differ markedly from the MP2/aug-ccpV(T+d)Z-, CCSD(T)/cc-pVTZ//MP2/aug-cc-pV(T+d)Z-, and CCSD(T)/cc-pV(T+d)Z//B3LYP/aug-cc-pV(T+d)Z-calculated energies. Conformers 1 and 2 are close in energy for both the B3LYP and MP2 calculations and differ only in the sign of the dihedral angle ∠H2S2CC or the relative orientation of the two SH bonds. This small change in conformation leads to a very small change in energy. Conformers 3 and 4 are predicted to have higher energies than conformers 1 and 2 with the B3LYP method. However, at the MP2, CCSD(T), and QB3 levels this is reversed. In the higher level calculations, conformer 3 has the lowest energy followed by conformer 4. The structural degeneracy is 4 for conformer 4 and 2 for conformer 3, and as the energy difference is relatively small, conformer 4 has higher room-temperature abundance than conformer 3. The abundance of conformer 3 and 4 is about 30% each with conformer 2 at 20% and conformers 1 and 5 each at about 10%. Previous studies have indicated that the anti conformation (conformers 1, 2, and 5) should be stabilized with respect to the gauche conformation (conformers 3 and 4) by approximately 0.8 kcal/mol.82 Our B3LYP/aug-cc-pV(T+d)Z calculations agree with this, whereas the higher levels predict that conformers 3 and 4, which have gauche conformations, have the lowest energy. Observed and Calculated Spectra. The experimentally observed spectrum of the fundamental XH-stretching region of
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TABLE 1: Calculated Dihedral Angles (deg), Relative Energies (cm-1), and Abundances (%) of 2-Mercaptoethanola ∆E conformer (degeneracy)
∠HOCCa
∠HSCCa
∠OCCSa
B3LYP/aVTZb
MP2a
CCSD(T)/aVTZc
CBS-QB3+ ZPVE
∆G CBS-QB3(298K)
Abd
1 (4) 2 (4) 3 (4) 4 (4) 5 (4)
51.7 66.5 172 -71.5 -71.5
-67.2 82.7 55.6 47.9 65.8
-58.6 -60.2 -65.8 -62.3 -179
0 190 409 543 490
0 204 419 582 648
0 175 429 553 610
0 124 342 557 553
0 32 291 552 473
44 38 11 3 4
a The MP2/aug-cc-pV(T+d)Z-optimized structure and energy. b The B3LYP/aug-cc-pV(T+d)Z energy and optimized structure. c Calculated with the CCSD(T)/aug-cc-p(VTZ) method at the MP2/aug-cc-pV(T+d)Z-optimized geometry. d Boltzmann distribution from the CBS-QB3 ∆G energies at 298 K including the structural degeneracies.
TABLE 2: Calculated Dihedral Angles (deg), Relative Energies (cm-1), and Abundances (%) of 1,2-Ethanedithiola ∆E conformer (degeneracy)
∠ H1S1CCa
∠ H 2 S 2C a
∠ S1CCS2a
B3LYP/aVTZb
MP2a
CCSD(T)/aVTZc
CBS-QB3 + ZPVE
∆G CBS-QB3 (298K)
Abd
1 (2) 2 (2) 3 (2) 4 (4) 5 (4)
65.6 66.5 -73.9 -68.3 -179
65.6 -66.5 -73.9 56.7 67.1
176 -180 66.2 -65.7 179
0 -1 167 213 409
156 157 0 66 542
85 87 0 85 461
87 95 0 95 445
220 94 0 136 338
10 19 29 31 11
a The MP2/aug-cc-pV(T+d)Z-optimized structure and energy. b The B3LYP/aug-cc-pV(T+d)Z energy and optimized structure. c Calculated with the CCSD(T)/aug-cc-p(VTZ) method at the MP2/aug-cc-pV(T+d)Z-optimized geometry. d Boltzmann distribution from the CBS-QB3 ∆G energies at 298 K including the structural degeneracies.
2ME is shown in Figure 2, which clearly shows the aforementioned lack of intensity for the SH-stretching transitions when compared to the CH- and OH-stretching bands of the same species.59 In Figure 3 we show the ∆VSH ) 1 and 2 stretching transitions of 2ME. Due to the low intensity of the SH-stretching transition, we were not able to observe any higher overtones of this transition. The inherently weak SH-stretching intensity is illustrated in the top trace of Figure 3, where the pure ∆VSH ) 2 transition is overshadowed by a combination band to the red. The ∆VSH ) 3 transition is predicted to lie slightly to the blue of the ∆VOH ) 2 transition and is probably overwhelmed by the tail of the OH-stretching transitions. Table 3 lists the observed SH-stretching transitions of 2ME. We observe three peaks in the fundamental SH-stretching band of 2ME. This is more clearly shown in Figure 4, which shows the fundamental SH-stretching band of 2ME at 297.5 and 373 K. At the higher temperature, it is easier to observe the two peaks to the blue of the main structure in the band. We attribute these two peaks at 2593 and 2600 cm-1 to the SH stretch of conformers 2 and 3 of 2ME. From the calculated structures, we would expect conformers 3 and 4 of 2ME to exhibit a redshift in the SH-stretch, as the SH group is positioned in such a way that could facilitate a SH · · · O hydrogen-bond-like interaction. This may be observable in the higher overtones due to the divergence of the different SH-stretching transitions of the different conformers. The relatively small room-temperature Boltzmann populations of these conformers should preclude observation of these bands. However, in Figure 5, we clearly observed additional peaks in the ∆VOH ) 2 and 3 regions. In the ∆VOH ) 2 region we clearly see four OH-stretching bands that we assign to four different conformers. We have recorded the ∆VOH ) 3 region of 2ME with both conventional long-pass and photoacoustic spectroscopy. The ICL-PAS technique gives a higher spectral resolution than the conventional technique, yet both PAS and conventional ∆VOH ) 3 spectra look similar. In the ∆VOH ) 3 region we also assign four conformers. The second band seems to be split into two peaks, likely due to a
Figure 1. The five lowest energy conformers of 2ME (left) and EDT (right).
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Miller et al. TABLE 3: Observed and Calculated SH-Stretching Transitions of 2-Mercaptoethanola calculated ∆VSH
observed ν (cm-1)
νHarmionic NM (cm-1)b
νAO LM (cm-1)
1 1 1 2 2
2586 2593 2600 5073 5081
2583 2598 2602 -
2568 2586 2592 5040 5083
assignment conformer conformer conformer conformer conformer
1 2 3 1 2
a Calculated using the B3LYP/aug-cc-pV(T+d)Z method and basis set. b A scaling factor of 0.97 has been applied.
Figure 2. The fundamental stretching region of 2ME. Note the different intensities of the SH-, CH-, and OH-stretching oscillators.
Figure 4. The fundamental SH stretch of 2ME at 297.5 and 373 K.
Figure 3. The fundamental SH stretch (bottom trace) and the first overtone of the SH stretch (top trace) of 2ME.
resonance, with a center energy of 10 314 cm-1 (see Table 4). The local mode (LM) anharmonic oscillator (AO) calculated vibrational overtone intensities for the OH stretch of 2ME are given at the MP2/aug-cc-pV(T+d)Z and B3LYP/aug-ccpVT(+d)Z levels of theory in the Supporting Information, Tables S5 and S6, respectively. The calculated intrinsic intensities of conformers 3 and 4 are higher than those of conformers 1 and 2, but not sufficiently high as to compensate for their lower abundance. Thus, we expect the true Boltzmann populations or intensities of conformers 3 and 4 to be higher than what is stated in Tables 1, S5, and S6, respectively. We have found that assigning individual peaks in the ∆VOH ) 1 region of 2ME is not possible due to the proximity of the bands to each other, the rotational structure of the bands making spectral deconvo-
lution difficult and the probable contributions to the OHstretching region from combination bands. In Figure 6, we show the ∆VSH ) 1-4 SH-stretching transitions of EDT. The observed energies are given in Table 5. In the fundamental SH-stretching region, we observe a number of overlapping rotational bands with a number of Q-branches visible and indicative of the presence of a number of conformers. This is expected on the basis of the two thiol groups in each conformer and the relatively similar abundance of four of the conformers; see Table 2. The overall full width at half-maximum (fwhm) of the band of the first overtone (∆VSH ) 2) is similar to that in the fundamental region, approximately 30 cm-1. This is a bit surprising, as frequencies of XH-stretching transitions usually differ more the higher the excitation (eq 3). We were able to detect the ∆VSH ) 3 and 4 transitions of EDT, even though these transitions are very weak. The fwhm of these SH-stretching bands are, like ∆VSH ) 1 and 2, also approximately 30 cm-1. Thus the individual SH-stretching transitions that we expect from the different conformers and that were clearly evident in the ∆VSH ) 1 spectrum have very similar
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Figure 5. The fundamental, first, and second overtone OH-stretching transitions (∆VOH ) 1-3) of 2ME. The abscissa of each region is 498 cm-1 and centered on the OH-stretching region. The bottom trace of the ∆VOH ) 3 spectrum was recorded with conventional techniques and the top trace with ICL-PAS.
Figure 6. The fundamental, first, second, and third overtone SHstretching transitions (∆VSH ) 1-4) of EDT. The abscissa of each region is 200 cm-1 and centered on the transition.
TABLE 4: Observed and Calculated OH-Stretching Transitions of 2-Mercaptoethanola
TABLE 5: Observed and Calculated SH-Stretching Transitions of 1,2-Ethanedithiola calculated
calculated
νHarmonic NM (cm-1)b
∆VOH
observed ν (cm-1)
νHarmonic NM (cm-1)b
νLM AO (cm-1)
assignment
1 1 1 1 2 2 2 2 3 3 3 3 3
-c -c -c -c 6989 7049 7150 7190 10188 10296 10349 10469 10532
3630 3665 3693 3716 -
3572 3622 3645 3668 6972 7091 7128 7178 10200 10407 10450 10528
conformer conformer conformer conformer conformer conformer conformer conformer conformer conformer conformer conformer
1 2 4 3 1 2 4 3 1 2 4 3
a
Calculated with B3LYP/aug-cc-pV(T+d)Z normal-mode frequency calculations or B3LYP/aug-cc-pV(T+d)Z calculated local mode parameters and dipole moment functions. b A scaling factor of 0.97 has been applied. c The ∆VOH ) 1 manifold is too complicated and the OH-stretching energies sit too close to make reliable assignments from the observed spectrum.
vibrational frequencies and anharmonicities. The observed ∆VCH ) 1, 2, 3, and 4 regions of EDT are available in the Supporting Information. We present B3LYP/aug-cc-pV(T+d)Z harmonic frequencies in Tables 3, 4, and 5 for the SH stretch of 2ME, the OH stretch of 2ME, and the SH stretches of EDT, respectively. The B3LYP-
∆VSH 1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4
observed ν (cm-1) 2583 2588 2588 2592 ∼5080 ∼5089 7476
9772
νLM AO (cm-1)
S1H1
S2H2
S1H1
S2H2
assignment
2583 2588 2588 2590 2588 -
2585 2588c 2588c 2593 2599 -
2571 2572 2573 2578 2584 5051 5062 5049 5062 5074 7426 7434 7450 7434 7471 9707 9721 9744 9725 9774
2572 -c -c -c 2575 -c -c 5053 -c 5048 -c -c -c 7442 7447 -c -c -c 9739 9744
conformer 4 conformer 1 conformer 2 conformer 3 conformer 5 conformer 1 conformer 2 conformer 4 conformer 3 conformer 5 conformer 1 conformer 2 conformer 3 conformer 4 conformer 5 conformer 1 conformer 2 conformer 3 conformer 4 conformer 5
a Calculated with B3LYP/aug-cc-pV(T+d)Z normal-mode frequency calculations or B3LYP/aug-cc-pV(T+d)Z-calculated local mode parameters and dipole moment functions. b A scaling factor of 0.97 has been applied. c S1H1 ) S2H2 due to symmetry.
calculated frequencies, scaled by 0.97, match the observed XHstretching frequencies to within a few wavenumbers. Also presented in Tables 3, 4 and 5 are the B3LYP/aug-cc-pV(T+d)Z
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local mode anharmonic oscillator results for the SH and OH stretches studied in the present work. For the SH stretch of 2ME (Table 3), we find that for the ∆VSH ) 1 region the local mode anharmonic oscillator results are slightly better than the unscaled normal mode harmonic oscillator results. The local mode anharmonic oscillator calculated ∆VSH ) 2 energy for conformer 1 of 2ME is lower than the observed band by 33 cm-1. For conformer 2, we calculate an energy that is much closer to the experimentally observed band. The local mode anharmonic oscillator calculations for the OH stretch of 2ME are compared to the observed energies of the ∆VOH ) 2 and 3 regions in Table 4. We find a reasonable agreement between the B3LYP/augcc-pV(T+d)Z local mode anharmonic oscillator calculations and the experimental spectra. Comparing the calculated results of the SH stretches to the experimental spectra of EDT is complicated by the large extent of overlap present particularly in the higher overtones. In the fundamental region, the B3LYP/ aug-cc-pV(T+d)Z local mode anharmonic oscillator results do not agree with the experimental results as well as the scaled B3LYP/aug-cc-pV(T+d)Z normal mode harmonic oscillator results do. Comparing both the calculated normal mode harmonic oscillator and local mode anharmonic oscillator results with the experimentally observed energies has resulted in the assignments given in Tables 3-5. Tables with frequency and intensity results of both normal mode (NM) and LM AO calculations at the B3LYP/aug-ccpV(T+d)Z and MP2/aug-cc-pV(T+d)Z levels are available in the Supporting Information (Tables S1-S20). Hydrogen Bonding, van der Waals Radii and Atoms in Molecules. Bultinck et al. have stated that there is intramolecular SH · · · S hydrogen bonding present in EDT.81 They state that the hydrogen bonding in EDT is rather weak in comparison to that found in ethylene glycol, based on calculations of hydrogenbond distance and van der Waals radii. Using the approximate method published by Buemi, where the energy of the hydrogenbonded conformer is compared with the energy of a conformer where the proton donor moiety is turned away into a trans position with the C-C bond, they find values of the hydrogenbond strength to be 725 cm-1 for what we have called conformer 4 and 991 cm-1 for what we call conformer 3. They state that conformer 3 has two SH · · · S hydrogen bonds (∼500 cm-1 each SH · · · S hydrogen bond).83 Barkowski et al. also conclude that there is a weak intramolecular hydrogen bond in EDT by using the van der Waals radii argument.82 They observe the distance of the hydrogen bond to be 0.4 Å shorter than the sum of the van der Waals radii (3.05 Å). Barkowski et al. predict the energy of the intramolecular SH---S bond in EDT to be approximately 350 cm-1 (∼1 kcal mol-1) by comparing the gauche conformer of EDT, which is similar to our conformer 4, with the isoelectronic molecule 1,2-dichloroethane. Atoms in molecules (AIM) was used as implemented in Gaussian 09 to calculate the electron density distribution for all the calculated conformers of 2ME and EDT at the M06-2x/ aug-cc-pV(T+d)Z level of theory. The AIM calculation failed to detect (3, -1) bond critical points between the OH or SH groups and the S or O atoms.77,84 The data from the AIM calculation is available in the Supporting Information. The lack of a (3, -1) bond critical point in ethylene glycol is a contentious issue that ultimately comes down to the definition of a hydrogen bond. We do not see any (3, -1) bond critical points between our interacting moieties in 2ME and EDT. However, we do observe a red-shift in the OH-stretching spectra of 2ME and believe this red-shift is the spectral manifestation of a weak hydrogen-bond-like interaction.
Miller et al. TABLE 6: Calculated Hydrogen-Bond Distances (Å) for Selected Conformers of 2ME and EDTa MP2/aug-cc-pV(T+d)Z
B3LYP/aug-cc-pV(T+d)Z
conformer
R(XH · · · S)
R(XH-S)
2ME 1 2ME 3 EDT 4
2.57
R(XH-O)
2.68 2.51
2.76
R(XH-O) 2.67
2.93
a On the basis of van der Waals radii arguments, an intramolecular hydrogen bond is expected to form if the hydrogen bond distance is less than 3.05 Å with a sulfur atom as acceptor atom and 2.60 Å with an oxygen acceptor atom.85
To test whether we should expect to observe a hydrogenbond-like interaction in our experimental spectra, the proton donor XH bond must be orientated toward an area of lone-pair electron density on the proton acceptor and have a distance between the proton and the proton acceptor that is less than the sum of the two van der Waals radii for those atoms. We have analyzed the calculated distances between the donor proton and the acceptor atom for the conformers of 2ME and EDT that adopt a possible intramolecular hydrogen-bond-like configuration. These values are given in Table 6. The commonly accepted van der Waals radii, deduced by Pauling, are 3.05 Å for an XH · · · S hydrogen bond and 2.60 Å for an XH · · · O hydrogen bond.85 We would expect to observe an OH · · · S intramolecular hydrogen-bond-like interaction in conformer 1 of 2ME; conformer 2 is calculated to be within interaction distance but does not appear to be positioned favorably with regard to the structures shown in Figure 1. We would expect to see marginal SH · · · O hydrogen-bond-like interactions in conformer 3 of 2ME because our MP2/aug-cc-pV(T+d)Z calculated structure of conformer 3 is within range and does appear to be pointing toward an area of electron density on the O atom. We expect that there would be SH · · · S intramolecular hydrogen-bond-like interactions in conformer 4 of EDT as the interaction distance is calculated to be shorter than the van der Waals radii by about 0.29 Å for MP2/aug-cc-pV(T+d)Z and 0.12 Å for B3LYP/augcc-pV(T+d)Z and the SH group is directed toward an area of S lone pair electron density. We do not expect to see any SH · · · S hydrogen-bond-like interaction in conformer 3, which is contrary to the findings of Bultinck et al.81 It would be an interesting extension of this study to record spectra of 1,3-propanedithiol, where the extra CH2 group in the carbon backbone should lead to a more favorable conformation for SH · · · S hydrogen bonding.14,86 Comparison of OH Spectra of 2-Mercaptoethanol with Those of Ethylene Glycol. The strength of a hydrogen bond is routinely related to how far the hydrogen bonded band is redshifted from the non-hydrogen-bonded (free) band in the vibrational spectrum. This method is particularly well-suited in spectra of molecules that have more than one conformation at room temperature, as the free band can be taken from a conformer that does not exhibit any hydrogen bonding. In the case of EDT, we can be confident in stating that there is no SH · · · S intramolecular hydrogen-bond-like interactions exhibited in the spectra. This is evidenced in both the observed spectra and the ab initio calculations. We do not observe significant splitting in the SH-stretching bands of 2ME. In ∆VSH ) 1 there appears to be at least three conformers, whereas in ∆VSH ) 2 the signal is weak and it is difficult to assign more than two transitions. Our anharmonic oscillator local mode calculations suggest three bands with intensity ratios of approximately 6:2:1, separated by about 25 cm-1 each in the ∆VSH ) 2 region. Conversely, we see band
Overtone Spectra of 2ME and EDT
J. Phys. Chem. A, Vol. 114, No. 48, 2010 12699 Conclusion
Figure 7. The ∆VOH ) 3 spectra of 2ME (black trace) and ethylene glycol (EG, red trace).
splitting and spreading from the OH stretch of 2ME. This is evident in both the experimentally observed spectra and the calculations. Here we see experimentally determined frequencies that span approximately 350 cm-1 in the ∆VOH ) 3 region. Snyder et al. modeled OH · · · O, OH · · · S, SH · · · O, and SH · · · S hydrogen bonding using constrained geometries like those typically found in intramolecular hydrogen-bonded cases.87 They calculated that the intramolecular OH · · · S bond strength will generally be larger than the intramolecular OH · · · O bond strength. Correspondingly, frequency shifts associated with intramolecular OH · · · S hydrogen bonds are considerably larger than those of the OH · · · O intramolecular hydrogen bonds. Howard and Kjaergaard found that the methanol-dimethyl sulfide (MeOH-DMS) complex resulted in a slightly larger redshift of the OH-stretching transition in the fundamental region than the corresponding methanol-dimethyl ether (MeOH-DME) complex.17 However, Howard and Kjaergaard measured the enthalpy of hydrogen-bond formation to be about 20% smaller in the MeOH-DMS complex.17 This result is supported by the recent calculation that found that the hydrogen-bond strength between p-cresol and diethyl sulfide is about 80% that of the hydrogen-bond strength in the p-cresol-diethyl ether complex, even though the OH bond elongation, and correspondingly the experimentally observed OH-stretching red-shift, is similar between the two complexes.53 We observe a OH-stretching red-shift in conformer 1 of 2ME of between 160 and 200 cm-1 in the first overtone and between 270 and 350 cm-1 in the ∆VOH ) 3 stretching region when compared with conformers 3 and 4. One of the most studied simple molecules with vicinal XH groups is ethylene glycol.2 Howard et al. reported the red-shift of the OH · · · O hydrogen bond in ethylene glycol to be between 167 and 100 cm-1 in the ∆VOH ) 3 transition.2 We compare the ∆VOH ) 3 spectra of 2ME and EG as recorded by Howard et al. in Figure 7. Figure 7 shows that the ∆VOH ) 3 region of 2ME spans a much larger region compared to EG and we see that the OH-stretching redshift in 2ME is larger than that of EG. From this, we infer that the OH · · · S intramolecular hydrogen-bond-like interaction in 2ME is similar in strength to the one found in ethylene glycol.17,53
Vapor-phase vibrational overtone spectra have been recorded in the ∆VOH ) 1-3 and ∆VSH ) 1 and 2 regions for 2-mercaptoethanol (2ME) and in the ∆VSH ) 1-4 and ∆VCH ) 1-4 regions for 1,2-ethanedithiol (EDT) to investigate OH · · · S, SH · · · O, and SH · · · S intramolecular hydrogen-bond-like interactions in these molecules. DFT and ab initio methods, using both normal mode and local mode anharmonic oscillator models, were used to help interpret our observed spectra. The agreement between our calculated and experimentally observed data is reasonable. We calculate five stable conformers for both 2ME and EDT, and for both species a gauche arrangement around the C-C bond is energetically favored. The most stable conformer of 2ME is capable of forming a weak hydrogen-bond-like interaction from the O-H to the S. The experimental OH-stretching vibrational spectra support this hypothesis. We observe four bands, for which the separation increases as higher energy OHstretching overtones are excited. One of these bands diverges further to the red than the other three. We assign this band as belonging to a conformer where the OH group takes part in a OH · · · S hydrogen-bond-like interaction. To gain insight into the strength of the OH · · · S hydrogen-bond-like interaction in 2ME, we compare the red-shift observed in the ∆VOH ) 3 region with the red-shift in the same region of ethylene glycol. We find the OH · · · S intramolecular hydrogen-bond-like interaction to have a larger red-shift than the OH · · · O intramolecular hydrogen-bond-like interaction, which indicates an interaction of similar strength. We do not observe any SH · · · O intramolecular hydrogen-bond-like interaction in our vibrational overtone spectra of 2ME. For EDT, we do not observe any interaction between the two thiol groups. The van der Waals radii calculations show that there could be some SH · · · S hydrogen-bond-like interaction in the most stable conformer, conformer 4; however, we do not observe any significant red-shifting in our SH-stretching spectra. Acknowledgment. We thank Dr. K. Takahashi, for helpful discussions. M.E.D. acknowledges funding from the NASA NESSF program. We acknowledge funding from the NSF and the Marsden Fund administered by the Royal Society of New Zealand. Supporting Information Available: Spectra showing the ∆VCH ) 2 and 3 transitions of 2ME and the ∆VCH ) 1, 2, 3, and 4 transitions of 1,2-ethanedithiol; tables containing the MP2/ aug-cc-pV(T+d)Z and B3LYP/aug-cc-pV(T+d)Z calculated harmonic frequencies and intensities for all conformers of 2ME and EDT, calculated local mode anharmonic oscillator intensities, energies and local mode parameters for the OH- and SHstretching transitions in 2ME and EDT; AIM data for all conformers at the M06-2x/aug-cc-pV(T+d)Z level of theory and basis set; Z-matrices for five conformers of each species are included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rosei, M. A. Physiol. Chem. Phys. Med. NMR 1988, 20, 189. (2) Howard, D. L.; Jorgensen, P.; Kjaergaard, H. G. J. Am. Chem. Soc. 2005, 127, 17096. (3) Klein, R. A. Chem. Phys. Lett. 2006, 429, 633. (4) Chopra, D.; Row, T. N. G.; Arunan, E.; Klein, R. A. J. Mol. Struct. 2010, 964, 126. (5) Howard, D. L.; Kjaergaard, H. G. J. Phys. Chem. A 2006, 110, 10245. (6) Klein, R. A. J. Comput. Chem. 2003, 24, 1120.
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