Torsional Fundamentals Research Articles

Theoretical characterization of dimethyl carbonate at low temperatures.

Highly correlated ab initio methods (CCSD(T) and RCCSD(T)-F12) are employed for the spectroscopic characterization of the gas phase of dimethyl carbonate (DMC) at low temperatures. DMC, a relevant molecule for atmospheric and astrochemical studies, shows only two conformers, cis-cis and trans-cis, respectively, of C2v and Cs symmetries. cis-cis-DMC represents the most stable form. Using RCCSD(T)-F12 theory, the two sets of equilibrium rotational constants have been computed to be Ae = 10 493.15 MHz, Be = 2399.22 MHz, and Ce = 2001.78 MHz (cis-cis) and to be Ae = 6585.16 MHz, Be = 3009.04 MHz, and Ce = 2120.36 MHz (trans-cis). Centrifugal distortions constants and anharmonic frequencies for all of the vibrational modes are provided. Fermi displacements are predicted. The minimum energy pathway for the cis-cis → trans-cis interconversion process is restricted by a barrier of ∼3500 cm(-1). DMC displays internal rotation of two methyl groups. If the nonrigidity is considered, the molecule can be classified in the G36 (cis-cis) and the G18 (trans-cis) symmetry groups. For cis-cis-DMC, both internal tops are equivalent, and the torsional motions are restricted by V3 potential energy barriers of 384.7 cm(-1). trans-cis-DMC shows two different V3 barriers of 631.53 and 382.6 cm(-1). The far-infrared spectra linked to the torsional motion of both conformers are analyzed independently using a variational procedure and a two-dimensional flexible model. In cis-cis-DMC, the ground vibrational state splits into nine components: one nondegenerate, 0.000 cm(-1) (A1), four quadruply degenerate, 0.012 cm(-1) (G), and four doubly degenerate 0.024 cm(-1) (E1 and E3). The methyl torsional fundamentals are obtained to lie at 140.274 cm(-1) (ν15) and 132.564 cm(-1) (ν30).

Conformational Stability from Rare Gas Solutions, r0 Structural Parameters, Barriers to Internal Rotation, and Ab initio Calculations for Vinyl Silyl Fluoride

The Raman (3300-10 cm(-1)) and infrared (3300-40 cm(-1)) spectra of gaseous and solid vinyl silyl fluoride, CH(2)=CHSiH(2)F, have been recorded. Raman spectrum of the liquid has also been recorded and depolarization values obtained. Variable-temperature studies of the infrared spectra of the sample dissolved in liquid krypton (-110 to -150 degrees C) and liquid xenon (-60 to -100 degrees C) have been carried out. From these studies, the enthalpy difference has been determined to be 76 +/- 7 cm(-1) (0.91 +/- 0.08 kJ/mol) from the krypton solutions and 69 +/- 7 cm(-1) (0.82 +/- 0.08 kJ/mol) from the xenon solutions, with the gauche conformer the more stable form. From the far-infrared spectrum of the gas, the asymmetric torsional fundamentals for the cis and gauche conformers have been observed at 102.34 and 86.56 cm(-1), respectively, with each having several "hot bands" falling to lower frequencies. From these frequencies along with the experimentally determined conformational enthalpy difference, as well as the gauche skeletal dihedral angle, the potential function governing the conformational interchange has been determined with the following Fourier cosine potential coefficients: V(1) = -80 +/- 11, V(2) = -42 +/- 15, V(3) = 622 +/- 5, V(4) = 34 +/- 5, and V(6) = -31 +/- 2 cm(-1). The gauche-to-cis and gauche-to-gauche barriers are 664 cm(-1) (7.94 kJ/mol) and 608 cm(-1) (7.27 kJ/mol), respectively. Complete vibrational assignments are provided for both conformers. In addition, equilibrium geometries and electronic energies have been determined for both rotamers from ab initio calculations using restricted Hartree-Fock and Møller-Plesset perturbation method to the second order (MP2), as well as density functional theory by the B3LYP methods, employing a number of basis sets up to 6-311+G(2df,2pd). All levels of calculation predict the gauche conformer to be the more stable form. By systematically adjusting the ab initio predicted structural values to fit the previously reported microwave rotational constants, adjusted r(0) parameters have been obtained for both conformers. These values are compared to those for the corresponding chloride and methyl compounds. The spectroscopic and theoretical results are discussed and compared to the corresponding quantities for some similar molecules.

Conformational stabilities and structural parameters of (CH 3) nCH 3− n CFO molecules

Variable temperature (−60 to −100 °C) studies of the infrared spectra (3500–400 cm −1) of propionyl fluoride (CH 3CH 2CFO) and 2-methylpropionyl fluoride ((CH 3) 2CHCFO), dissolved in liquid xenon have been recorded. From these data, the enthalpy difference has been determined to be 329±33 cm −1 (3.94±0.39 kJ/mol) for propionyl fluoride with the trans conformer (methyl group eclipsing the oxygen atom) more stable than the gauche form. For 2-methylpropionyl fluoride, the enthalpy difference has been determined to be 297±30 cm −1 (3.55±0.36 kJ/mol) with the gauche conformer (methyl group eclipsing the oxygen atom) more stable than the trans form. From these Δ H values along with assigned torsional fundamentals for both conformers and accompanying “hot bands” the potential functions governing the conformational interchange have been calculated. Utilizing the infrared data from the xenon solution and ab initio frequency predictions from MP2/6-31G * calculations, a few reassignments of the fundamentals have been made. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G ** from which structural parameters and conformational stabilities have been determined. Additionally, force constants, infrared intensities, Raman activities, depolarization ratios, and scaled vibrational frequencies have been determined from MP2/6-31G * calculations. Adjusted structural parameters have been obtained from combined ab initio predicted values and previously reported microwave data. These parameters are compared to those obtained from either the earlier microwave and/or electron diffraction studies. Similar ab initio calculations and structural parameter determinations have been carried out for acetyl fluoride (CH 3CFO) and trimethylacetyl fluoride ((CH 3) 3CCFO) and compared to the corresponding experimental results when appropriate.

Conformational Analysis, Barriers to Internal Rotation, Vibrational Assignment, and ab Initio Calculations of 3-Fluoro-1-butene

The far-infrared spectrum of gaseous 3-fluoro-1-butene, CH2CHC(CH3)FH, has been recorded at a resolution of 0.10 cm-1. The asymmetric torsional fundamentals of the most stable form HE (hydrogen ato...

Conformational Analysis, Barriers to Internal Rotation, ab Initio Calculations, and Vibrational Assignment of 4-Fluoro-1-butyne

The infrared spectra (3400−300 cm-1) of gaseous, xenon and krypton solutions, and solid state and the Raman spectra (3400−10 cm-1) of the liquid and solid states have been recorded for 4-fluoro-1-butyne, CH2FCH2C⋮CH. These data have been interpreted to show that the molecule exists in the anti conformation (the C−F bond is trans to the C⋮C bond) and the gauche forms in the vapor and liquid, but only the gauche conformer is present in the solid. From variable-temperature infrared studies of xenon and krypton solutions, the anti conformation has been determined to be more stable than the gauche form by 215 ± 22 cm-1 (2.57 ± 0.26 kJ/mol) and 170 ± 17 cm-1 (2.04 ± 0.2 kJ/mol), respectively. The asymmetric torsional fundamentals have been observed at 109.4 and 116.6 cm-1 for the more stable anti and the high-energy gauche conformers, respectively. From these data the asymmetric torsional potential function governing the internal rotation about the C−CH2F bond has been determined and the potential parameters are V1 = 438 ± 14, V2 = −157 ± 12, V3 = 1137 ± 5, and V4 = 17 ± 6 cm-1. This potential function is consistent with the anti to gauche and gauche to gauche barriers of 1142 and 1364 cm-1, respectively, and the dihedral angle FCCC for the gauche conformer of 64°. These data are compared to the corresponding quantities obtained from ab initio calculations, which predict the anti conformer to be the more stable form. Vibrational assignments for the 24 normal modes for both the anti and gauche conformers are proposed. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

Four-dimensional model calculation of torsional levels of cyclic water tetramer

Quantum four-dimensional model calculations of the coupled intermolecular torsional vibrations of the cyclic homodromic water tetramers (H2O)4 and (D2O)4 are presented, based on the analytical modEPEN4B potential energy surface [S. Graf and S. Leutwyler, J. Chem. Phys. 109, 5393 (1998), preceding paper] and a four-dimensional discrete variable representation approach. The lowest 50 torsional levels were calculated up to 420 and 500 cm−1 for (D2O)4 and (H2O)4, respectively. For both clusters, the torsional ground state is split by a synchronous O–H torsional inversion process, similar to inversion tunneling in ammonia, with calculated tunnel splittings of 21.8 and 0.000 12 MHz for (H2O)4 and (D2O)4, respectively. As for the cyclic water trimer and pentamer, the four torsional fundamentals of the tetramer lie above the torsional interconversion barriers, between 185–200 cm−1 for (D2O)4 and 229–242 cm−1 for (H2O)4, but also lie below the one-dimensional torsionally adiabatic barriers. The anharmonic fundamental frequencies lie both above and below the normal-mode frequencies, by up to 33%. Slightly above the fundamental torsional excitations, at 257–260 and 280–281 cm−1 for (H2O)4 and (D2O)4, respectively, lie four states corresponding to four versions of the {uudd} isomer, which form a pseudorotational manifold; the torsional interconversion occurs by a sequence of double O–H flips. Higher excited pseudorotational states are calculated up to a vibrational angular momentum of k=3. At ≈295 and ≈300 cm−1, a further group of eight states is found, corresponding to the eight permutationally equivalent versions of yet another isomer, the {uuud} structure. The four {uudd} and eight {uuud} states of (H2O)4 exhibit inverse isotope effects, and lie at lower energy than their (D2O)4 counterparts.

Conformational stability and structural parameters of CH 3CH 2CClO and other (CH 3) nCH 3− nCClO molecules

Variable temperature (−60 to −100°C) studies of the infrared spectra (3500 to 400 cm −1) of propionyl chloride, CH 3CH 2CClO, dissolved in liquid xenon have been recorded. From these data the enthalpy difference between the rotational isomers has been determined to be 505±32 cm −1 (1.44±0.09 kcal mol −1) with the trans conformer (methyl group eclipsing the oxygen atom) more stable than the gauche form. From this Δ H value, along with assigned torsional fundamentals for both conformers and accompanying “hot bands”, the potential function governing the conformational interchange was calculated. Utilizing the infrared data from the xenon solution, along with some new Raman data and ab initio frequency predictions from MP2/6-31G* calculations, a few reassignments of the fundamentals have been made. Ab initio calculations were carried out with several different basis sets up to MP2/6-311G** from which structural parameters and conformational stabilities were determined. These parameters are compared to those obtained from earlier microwave and electron diffraction data. Similar ab initio calculations have been carried out for CH 3CClO, (CH 3) 2CHCClO and (CH 3) 3CClO and these results are compared to the corresponding experimental results where appropriate.

Conformational studies of propenoyl fluoride in liquid xenon from temperature dependent FTIR spectra

-Variable temperature (-60°C to -100°C) infrared spectra (3500–400 cm -1) of propenoyl fluoride, CH2CHCFO, dissolved in liquid xenon have been recorded. From these data the 2 enthalpy difference has been determined to be 113±15 cm-1 (322±42 cal mol-1) with the anti conformer (carbonyl bond trans to the carbon-carbon double bond) more stable than the syn form. From this Δ H value along with assigned torsional fundamentals for both conformers and accompanying ‘hot bands’ the potential function governing the conformational interchange has been calculated. Utilizing the infrared data from the xenon solution, some new Raman data, and ab initio frequency predictions from MP2/6-31G** calculations, a few reassignments of the fundamentals have been made. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311++G** from which structural parameters and conformational stabilities have been determined. With all the basis sets, the anti conformer is predicted to be the more stable conformer, which is in agreement with the experimental results. These spectroscopic and theoretical results are compared with the corresponding quantities for some similar molecules.

Conformational studies of propenoyl fluoride in liquid xenon from temperature dependent FTIR spectra

-Variable temperature (-60°C to -100°C) infrared spectra (3500–400 cm -1) of propenoyl fluoride, CH2CHCFO, dissolved in liquid xenon have been recorded. From these data the 2 enthalpy difference has been determined to be 113±15 cm-1 (322±42 cal mol-1) with the anti conformer (carbonyl bond trans to the carbon-carbon double bond) more stable than the syn form. From this Δ H value along with assigned torsional fundamentals for both conformers and accompanying ‘hot bands’ the potential function governing the conformational interchange has been calculated. Utilizing the infrared data from the xenon solution, some new Raman data, and ab initio frequency predictions from MP2/6-31G** calculations, a few reassignments of the fundamentals have been made. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311++G** from which structural parameters and conformational stabilities have been determined. With all the basis sets, the anti conformer is predicted to be the more stable conformer, which is in agreement with the experimental results. These spectroscopic and theoretical results are compared with the corresponding quantities for some similar molecules.

Conformational studies of propenoyl chloride in liquid xenon from temperature dependent FT-IR spectra

Variable temperature (−60 to −100°C) studies of the infrared spectra (3500 to 400 cm −1) of propenoyl chloride, CH 2CHCClO, dissolved in liquid xenon have been recorded. From these data the enthalpy difference has been determined to be 139 ± 8 cm −1 (397 ± 23 cal/mol) with the anti conformer (carbonyl bond trans to the carbon-carbon double bond) more stable than the syn form. From this ΔH value along with assigned torsional fundamentals and accompanying “hot bands” the potential function governing the conformational interchange has been calculated. Utilizing the new infrared data from the xenon solution, some new Raman data, and ab initio predictions from MP2/6–31G ∗ calculations a few reassignments of the fundamentals have been made. Ab initio calculations have been carried out with several different basis sets up to MP2/6–311G ∗∗ from which structural parameters and conformational stabilities have been determined. With all the basis sets the syn conformer is predicted to be the more stable conformer which is inconsistent with the experimental results. These results are compared to the corresponding quantities for some similar molecules.

Raman and infrared spectra, conformational stability, barriers to internal rotation and ab initio calculations for 3-methyl-2-butenoyl fluoride

The Raman (3200 to 20 cm −1) spectra of liquid and solid and infrared (3200 to 400 cm −1) spectra of gaseous and solid 3-methyl-2-butenoyl fluoride (3,3-dimethylacryloyl fluoride), (CH 3) 2C=C(H)CFO, have been recorded. The far-infrared spectrum of the gas has also been recorded from 360 to 50 cm −1 at a resolution of 0.1 cm −1. The syn (s-cis) conformer is the only stable rotamer (two double bonds oriented cis to each other) present in the fluid at ambient temperature and solid states. The barriers for the syn (s-cis) to anti (s-trans) and anti to syn conformers are predicted to be 2498 and 1931 cm −1, respectively, with an energy difference of 567 cm −1 (1.62 kcal mol −1) from the RHF/6-31G* calculations. The energy difference was only slightly smaller, 487 cm −1 (1.39 kcal mol −1) with electron correlation, i.e. MP2/6-31G*. Barriers to internal rotation of the two non-equivalent methyl rotors with torsional fundamentals at 205 (trans methyl rotor) and 109 cm −1 were determined to be 920 cm −1 (2.63 kcal mol −1) and 304 cm −1 (0.87 kcal mol −1), respectively. A complete assignment of the normal vibrational modes is proposed. The structural parameters, force constants and vibrational frequencies for the syn conformer have been determined from ab initio gradient calculations employing the RHF/3-21G* and RHF/6-31G* basis sets along with electron correlation at the MP2/6-31G* level and the theoretical results are compared to the experimental values when appropriate. These results have been compared with the corresponding ones obtained for some similar molecules.

Raman and infrared spectra, vibrational assignment and ab initio calculations of trans‐2‐methyl‐2‐butenenitrile

AbstractThe Raman (3100‐10 cm−1) and infrared (3100‐400 cm−1) spectra of trans‐2‐methyl‐2‐butenenitrile [trans‐CH3CH =C(CH3)CN] were recorded for the gas, liquid and solid phases. Additionally, the far‐infrared (370‐60 cm−1) spectrum of the gas was recorded. These data were interpreted on the basis that only the trans (two methyl groups trans to each other) isomer is present on purification. Only one of the methyl torsional fundamentals was observed and it corresponds to the methyl group on the same carbon atom as the cyano group. The experimental barrier to internal rotation is 610 ± 30 cm−1 (1.74 ± 0.09 kcal mol−1). The structure, infrared intensities, Raman activities, depolarization ratios and vibrational wavenumbers were determined from ab initio calculations using the RHF/3‐21G, RHF/6‐31G* and MP2/6‐31G* basis sets. These results are compared with those obtained for some related molecules.

The microwave spectrum and potential function of nitrosoethane, CH 3CH 2NO

The potential function for internal rotation of the nitroso group in nitrosoethane, CH 3CH 2NO, has been determined. All available microwave data are fitted to the potential, V(α) = 1 2 ∑ n V n[1 − cos(nα)] where V 1 = (−15), V 2 = 63(2), V 3 = 389(1), V 4 = 135(1) and V 5 = 166(2) cm −1. Torsional fundamentals for the nitroso group (CN) have been measured by microwave relative intensity measurements for both rotamers: v t (cis) = 182(17) cm −1 and v t (gauche) = 61(19) cm −1. The energy difference between the two stable conformers has been determined to be ΔW(gauche - cis) = 2.1(4) kJ mol −1. The ground state torsional splitting for gauche-CH 3CH 2NO has been re-evaluated as ΔE ± = 153.5(4) MHz corresponding to a calculated value of the coupling constant T bc = 34.7 MHz. Accurate structural data have been used in the potential function calculations. The resulting potential is compared with those of some isoelectronic molecules.

Enhancement of Cα hydrogen vibrations in the resonance Raman spectra of amides

AbstractUltraviolet resonance Raman (UVRR) spectra of polypeptides and proteins exhibit a band at ca. 1390 cm−1 whose intensity is sensitive to helical content. The assignment of this band, designated amide S, has been contentious. To study the vibrational origin of amide S, relevant amide model compounds and their selectively deuterated iso‐topomers were synthesized. N‐Methylacetamide, N‐methylpropionamide and N‐methylisobutyramide possess a methyl, methylene and methine group, respectively, adjacent to the amide carbonyl. The UVRR spectrum of the natural abundance species show resonantly enhanced vibrational bands in the region 1350–1380 cm−1. These bands disappear on deuteration of the (C)Cα. hydrogen atoms, while the amide III modes shift up to a common position (ca. 1335 cm−1) and increase in intensity. These results suggest that amide S can be assigned to a (C)Cα hydrogen bending vibration, which acquires resonance intensity by vibrational mixing with the CN stretch of the nearby amide III mode. Variation of this mixing with the CαH/CO dihedral angle can explain the sensitivity of amide III to protein secondary structure. Additional UVRR and Fourier transform IR data reveal that (C)Cα‐deuteration has a smaller effect on the amide I, I′, II and II′ vibrational modes. Finally, it is established that UVRR spectra for NMA in H2O and D2O show an absence of torsional fundamentals or overtones. These results are interpreted as indicating an energy barrier to cxcited‐state isomerization, which argues against an alternative assignment of amide S to the amide V overtone.

Torsional spectra of molecules with two C3v rotors—XXV. Rotational and vibrational spectra, r0 structure, barriers to internal rotation and ab initio calculations for 2,2-difluoropropane

The microwave spectra of two isotopic species of 2,2-difluoropropane, (CH 3) 2 13CF 2 and (CD 3) 2 12CF 2, have been recorded from 26.5 to 40.0 GHz and the b-type R-branch transitions have been observed and assigned for the ground state. Utilizing the rotational transitions for these two isotopomers along with those for the normal species the following r o structural parameters have been determined: r(C-H) = 1.091±0.002Å, r(C-C) = 1.510 + 0.005 Å, r(C-F) = 1.378±0.005 Å, ∢CCH = 109.26±0.05°, ∢CCC = 116.44±0.54° and ∢FCF = 105.15±0.50°. The carbon-hydrogen distances have also been obtained from the frequency of the isolated carbon-hydrogen frequency in the IR spectrum. The far-IR spectra of 2,2-difluoropropane- d 0 and - d 6 in the gas were recorded at a resolution of 0.10 cm −1 and the torsional fundamentals, along with several hot band transitions, have been assigned. From these data the barrier to internal rotation of the coupled rotors is 1103±48cm −1 (3.15±0.14kcalmol −1). The IR spectra (3600–50cm −1) of gaseous and solid and the Raman spectra (3600–10 cm −1) of gaseous, liquid and solid (CD 3) 2CF 2 have been recorded. A complete vibrational assignment is proposed for both the normal and d 6 molecules based on depolarization values, relative intensities, isotopic shifts and normal coordinate calculations. The structural parameters, barriers to internal rotation and fundamental vibrational frequencies which have been determined experimentally are compared with those obtained from ab initio Hartree-Fock gradient calculations utilizing both the 3–21G and 6–31G* basis sets along with electron correlation at the MP2 level, and to the corresponding quantities obtained for some similar molecules.

Raman and infrared spectra, conformational stability, barriers to internal rotation,r 0 structural parameters, ab initio calculations, and vibrational assignment for vinyl chloroformate

The Raman (3200 to 10 cm−1) and infrared (3500 to 50 cm−1) spectra of vinyl chloroformate, H2C=CHOC(O)Cl, have been recorded for both the gas and solid. Additionally, the Raman spectrum of the liquid has been recorded, and depolarization ratios have been obtained. These data have been interpreted on the basis that the only stable conformation present at ambient temperature is thetrans-trans rotamer, where the firsttrans refers to the vinyl moiety relative to the O—CCl bond and the second to the C—Cl bond relative to the=C—O bond. Using harmonic rigid asymmetric top calculations, the infrared vapor phase contours for the C=O and the C=C stretch were predicted for thetrans-trans and for thecis-trans conformer, and were compared with experiment. For both fundamentals thetrans-trans hybrid reproduces the experimental contour, whereas thecis-trans contours fail to do so for both fundamentals. From far-infrared spectrum of the vapor obtained at 0.1 cm−1 resolution, the C(O)Cl andO-vinyl torsional fundamentals have been observed at 132 and 61 cm−1, respectively. Ther0 structural parameters have been obtained from a combination of ab initio calculated parameters with appropriate offset values and the fit of the microwave rotational constants for the two naturally occurring chlorine isotopes. The structure, barrier to internal rotation, and vibrational frequencies have been determined from ab initio Hartree-Fock gradient calculations, using the 3-21G* and 6-31G* basis sets. These results are compared to those obtained experimentally and to similar quantities for some related molecules.

Raman and infrared spectra, conformational stability, barriers to internal rotation, ab initio calculations and vibrational assignment for vinyl fluoroformate

AbstractThe Raman spectra (3200—10 cm−1) and infrared spectra (3500—50 cm−1) of vinyl fluoroformate, H2CCHOC(O)F, were recorded for both the gas and solid. Additionally, the Raman spectrum of the liquid was recorded and depolarization ratios were obtained. These data have been interpreted on the basis that the only stable conformation present at ambient temperature is the trans‐trans, where the first trans refers to the vinyl moiety relative to the OCFO bond and the second trans to the CF bond relative to the CO bond. Anharmonic rigid asymmetric top calculations were used to predict the CO and CC stretch infrared vapor‐phase contours for the trans‐trans and the cis‐trans conformers. For both conformers, the predicted CO stretch contour agrees with the experimentally observed contour. For the CC stretch, only the contour predicted for the trans‐trans conformer reproduces the experimental contour. From the far‐infrared spectrum of the vapor obtained at a resolution of 0.1 cm−1, the methoxy and O‐vinyl torsional fundamentals have been observed at 153 and 63 cm−1, respectively. The structure, barrier to internal rotation and vibrational frequencies were also determined from ab initio Hartree‐Fock gradient calculations using both the 3—21G and 6—31G* basis sets. These results are compared with theos obtained experimentally and to similar quantities for some related molecules.

Barrier to asymmetric internal rotation, conformational stability, vibrational spectra and assignments, and Ab Initio calculations of n‐butane‐d0, d5 and d10

AbstractThe asymmetric torsional potential function of n‐butane has been redefined based on a combination of results obtained from experimental and theoretical calculations. From the vibrational spectra of gaseous n‐butane below 500 cm−1 the asymmetric torsional fundamentals of the more stable s‐trans and high‐energy gauche conformations of n‐butane have been assigned at 121.28 and 116.60 cm−1, respectively, each with excited states falling to lower frequencies. Additionally, from variable‐temperature studies of the Raman spectra for n‐butane‐d0 and d10, the conformational enthalpy difference has been determined to be 381 ± 41 cm−1 (1089 ± 117 cal mol−1) and 369 ± 72 cm−1 (1055 ± 206 cal mol−1), respectively. Optimized structural parameters, obtained from ab initio methods, for the s‐trans and gauche conformers have been used to allow for structural relaxation during the asymmetric internal rotation and to place constraints on the determined potential function. The resulting potential coefficients are V1 = 584 ± 7, V2 = ‐96 ± 7, V3 = 1165 ± 3, V4 = 45 ± 3, V5 = ‐3 ± 2 and V6 = ‐40 + 1 and the s‐trans to gauche, gauche to gauche and gauche to s‐trans barriers are determined to be 1274, 1370 and 891 cm−1, respectively, with an enthalpy difference between the conformers of 383 ± 17 cm−1 (1095 ± 49 cal mol−1). This potential function is consistent with a dihedral angle for the gauche conformer of 65.4°. Additionally, this potential has a ‘syn’ barrier, which is the energy difference between the s‐trans minimum and the gauche to gauche transition state, of 1753 cm−1 (5.01 kcal mol−1), which is in excellent agreement with the most recent value of 1836 cm−1 (5.25 kcal mol−1) obtained from a very high‐level ab initio calculation. In order to provide a more complete description of the normal vibrational modes of n‐butane, normal coordinate analyses have been completed for n‐butane‐d0, ‐d5 and ‐d10 based on force constants from ab initio calculations. The infrared (3500‐40 cm−1) and Raman (3500‐10 cm−1) spectra of n‐butane‐d5 and ‐d10 have been obtained and complete vibrational assignments are provided. The structural parameters, conformational stabilities, barriers to internal rotation and vibrational frequencies which have been determined experientally are compared with those obtained by ab initio methods.

Far-infrared spectra and barriers to internal rotation for the CF 3CXO molecules where X = H, F, Cl, Br, CH 3 and CF 3

The far-infrared spectra (350-30 cm −1) of CF 3CHO,CF 3CFO, CF 3CClO, CF 3CBrO, CF 3CO(CH 3) and (CF 3) 2CO have been recorded at a resolution of 0.10 cm −1. The torsional fundamentals for the CF 3 rotors have been assigned at 66.2, 45.6, 41.8, 38.3, 36.1 and 53.8 (39.0) cm −1, respectively, where two frequencies are observed or the CF 3 torsional modes of (CF 3) 2CO, and each fundamental for all of the molecules has several accompanying “hot bands”. Utilizing these torsional data, along with reasonable structural parameters to calculate the kinetic constants, the periodic barriers to internal rotation have been determined. The barrier values for the CF 3 rotors with the statistical uncertainties are 292 ± 5, 381 ± 1, 434 ± 2, 429 ± 2, 295 ± 1, and 777 ± 5 cm −1 for these CF 3CXO molecules, where X  H, F, Cl, Br, CH 3 and CF 3, respectively. These results are compared to the methyl barriers in the corresponding series CH 3CXO.

Microwave and far infrared spectra, r structure, barriers to internal rotation, and a b i n i t i o calculations for 2-fluoropropane

The microwave spectra of five isotopic species of 2-fluoropropane, (CH3)CH2DCFH, (CH3)2CFD, (CH3)CD3CFH, (CD3)2CFD, and (CH3)213CFH, have been recorded from 12.4 to 39.7 GHz. The b- and c-type R-branch transitions have been observed and assigned for the ground state. Utilizing the rotational constants for these five isotopic species along with those reported earlier for the normal species the following r0 structural parameters have been determined: r(C–C) =1.522±0.007 Å, r(C–F)=1.398±0.013 Å, CCC =113.37±0.79°, and CCF=108.19±0.41°. All of the carbon–hydrogen parameters have also been determined from the rotational constants except for the r(C–Hsec) which was obtained from its frequency in the infrared spectrum. The far infrared spectra of 2-fluoropropane-d0, -d3 and -d7 in the gas phase were recorded with a resolution of 0.10 cm−1. Both torsional fundamentals along with several hot transitions were assigned for the three isotopic species. The barrier to internal rotation of the methyl rotors has been determined with the two-coupled rotor model to be 1149±69 cm−1 (3.29±0.12 kcal/mole). Both potential coupling terms, V33 and V′33, have been determined for the -d0, -d3 and -d7 isotopic species. The complete equilibrium geometry has been determined from ab initio Hartree–Fock gradient calculations employing both the 3-21G and 6-31G* basis sets. These results are compared to the corresponding quantities for some similar molecules.