Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.     

Microwave and quantum chemical study of propa-1,2-dienyl thiocyanate (H2C=C=CHSC triple bond N)

The microwave spectrum of propa-1,2-dienyl thiocyanate (H2C=C=CHSC triple bond N) has been investigated in the 24-40 and 50-80 GHz spectral regions. The spectrum of one conformer was assigned. This rotamer, which has a C-C-S-C dihedral angle of about 134 degrees from synperiplanar, is at least 2 kJ/mol more stable than any other form. Two vibrationally excited states assumed to belong to the first excited state of the C-S torsional vibration and to a low bending mode were assigned. Their frequencies were determined to be 62(20) and 155(30) cm-1, respectively. The microwave work has been augmented by ab initio calculations at the MP2/aug-cc-pVTZ and density functional theory calculations at the B3LYP/aug-cc-pVTZ level of theory. The B3LYP calculations are generally in better agreement with the observations than the MP2 calculations.     

Microwave spectrum and conformational composition of 2-chloroethylisocyanide

2-Chloroethylisocyanide (ClCH(2)CH(2)N≡C) has been synthesized, and its microwave spectrum has been investigated in the 20-97 GHz spectral region. The spectra of (35)Cl and (37)Cl isotopologues of two conformers have been assigned. The Cl-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second. The energy difference between the two forms has been obtained from relative intensity measurements. It was found that the antiperiplanar conformer is favored over the synclinal form by 4.3(8) kJ/mol. Quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with their experimental counterparts. The theoretical calculations correctly predict that the 2-chloroethylisocyanide exists as a mixture of an antiperiplanar and a synclinal conformer, with the former about 3.5 kJ/mol more stable than the latter. Both methods of calculations find that the antiperiplanar rotamer has a symmetry plane. The dihedral angle formed by the Cl-C-C-N link of atoms of the synclinal form is 67° according to the CCSD calculations. It is estimated from a comparison with the experimental rotational constants that this dihedral angle is uncertain by ±3°.     

High resolution millimeter-wave spectroscopy of cyclopropylphosphine-borane

The microwave spectrum of cyclopropylphosphine-borane, C(3)H(5)PH(2)-BH(3), has been investigated in the frequency range 150-195 GHz. The spectral assignment was supported by high level ab initio calculations. Two stable conformations have been predicted: the most stable antiperiplanar form and synclinal form that is higher in energy by 7.3 kJ/mol. In the observed spectra, only the most stable antiperiplanar (ap) form has been assigned. The analysis of the rotational spectra in the lowest excited vibrational states of the ap conformer has enabled determination of the potential function for the C-P torsional mode in the vicinity of equilibrium position. The barrier to internal rotation of the BH(3) top has been determined to be 9.616(15) kJ/mol and agrees well with quantum chemical calculations.     

Ã-X absorption of propargyl peroxy radical (H-C≡C-CH2OO·): a cavity ring-down spectroscopic and computational study

The ?-X electronic absorption spectrum of propargyl peroxy radical has been recorded at room temperature by cavity ring-down spectroscopy. Electronic structure calculations predict two isomeric forms, acetylenic and allenic, with two stable conformers for each. The acetylenic trans conformer, with a band origin at 7631.8 ± 0.1 cm(-1), is definitively assigned on the basis of ab initio calculations and rotational simulations, and possible assignments for the acetylenic gauche and allenic trans forms are given. A fourth form, allenic cis, is not observed. Simulations based on calculated torsional potentials predict that the allenic trans form will have a long, poorly resolved progression in the OOCC torsional vibration, consistent with experimental observations.     

Rotational spectrum of cis-cis HOONO

The pure rotational spectrum of cis-cis peroxynitrous acid, HOONO, has been observed. Over 220 transitions, sampling states up to J'=67 and Ka'=31, have been fitted with an rms uncertainty of 48.4 kHz. The experimentally determined rotational constants agree well with ab initio values for the cis-cis conformer, a five-membered ring formed by intramolecular hydrogen bonding. The small, positive inertial defect Delta=0.075667(60) amu A2 and lack of any observable torsional splittings in the spectrum indicate that cis-cis HOONO exists in a well-defined planar structure at room temperature.     

Torsional potential of 4,4'-bipyridine: ab initio analysis of dispersion and vibrational effects

Ab initio calculations using restricted Hartree-Fock, second-order M?ller-Plesset perturbation theory (MP2), density-functional theory (DFT), and coupled-cluster methods have been done to obtain the torsional potential-energy profile of the aza-aromatic molecule 4,4'-bipyridine. The torsional potential is evaluated adiabatically by fixing the normalized sum of the dihedral angles through the C-C inter-ring bond at several values along the torsional path and relaxing the remaining degrees of freedom. Previous discrepancies between MP2 and DFT internal rotation barrier heights are removed, and seen to be mostly due to the underestimation of the dispersion energy in the coplanar conformer by MP2 when using relatively small basis sets. The calculations indicate that the barrier height between the twisted global minimum and the 0 degrees conformer is around 1.5-1.8 kcal mol-1 while that corresponding to the 90 degrees one is about 2.0-2.2 kcal mol-1. This same relative energy ordering of the coplanar and perpendicular conformers was experimentally derived from nuclear magnetic resonance (NMR) measurements of 1H dipolar couplings on 4,4'-bipyridine solutions in a nematic liquid crystal, although the barrier heights are much lower than those estimated from NMR experiments in the gas phase. The DFT infrared spectrum and zero-point vibrational energy corrections to the torsional energy profile have also been calculated, the latter having a small influence on the torsional potential-energy profiles.     

Microwave spectrum and conformational composition of 3-fluoropropionitrile (FCH2CH2CN)

The microwave spectrum of 3-fluoropropionitrile, FCH(2)CH(2)C≡N, has been investigated in the whole 17-75 GHz spectral region. Selected portions of the spectrum in the 75-95 GHz have also been recorded. The microwave spectra of the ground state as well as of three vibrationally excited states of each of two conformers have been assigned. The spectra of the vibrationally excited states belong to the lowest torsional and bending vibrations. The F-C-C-C chain of atoms is exactly antiperiplanar in one of these rotamers and synclinal in the second conformer. The F-C-C-C dihedral angle is 65(2)° in the synclinal form. The energy difference between the two forms has been obtained from relative intensity measurements performed on microwave transitions. It was found that the antiperiplanar conformer is more stable than the synclinal form by 1.4(5) kJ/mol. It is argued that the gauche effect is a significant force in this compound. Quantum chemical calculations at the high CCSD(full)/cc-pVTZ, MP2(full)/cc-pVTZ, and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the theoretical predictions are in good agreement with experiment.     

Microwave spectrum and conformational composition of 2-fluoroethylisocyanide

The microwave spectrum of 2-fluoroethylisocyanide, FCH(2)CH(2)N≡C, has been investigated in the whole 50-120 GHz spectral region. Selected portions of the spectrum in the range of 18-50 GHz have also been recorded. The microwave spectra of the ground state and vibrationally excited states of two conformers have been assigned. Accurate spectroscopic constants have been derived from a large number of microwave transitions. The F-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second conformer. The energy difference between the two forms was obtained from relative intensity measurements. It was found that the synclinal conformer is favored over the antiperiplanar form by 0.7(5) kJ/mol. Quantum chemical calculations at the high CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory were performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with the experimental counterparts. The theoretical calculations correctly indicate that the F-C-C-N dihedral angle in the synclinal form is about 67° but underestimate the magnitude of the gauche effect and erroneously predict the antiperiplanar rotamer to be 1.3-1.6 kJ/mol more stable than the synclinal conformer.     

Conformational stability, structural parameters and vibrational assignment from variable temperature infrared spectra of krypton solutions and ab initio calculations of ethylisothiocyanate

Variable temperature (-105 to -150 degrees C) studies of the infrared spectra (3500-400 cm(-1)) of ethylisothiocyanate, CH(3)CH(2)NCS, dissolved in liquid krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate a single conformer in all physical states with a large number of molecules in the gas phase at ambient temperature in excited states of the CN torsional mode which has a very low barrier to conformational interchange. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with valence and core electron correlation using a variety of basis sets up to 6-311+G(2df,2pd). With the smaller basis sets up to 6-311+G(d,p) and cc-PVDZ, the cis conformer is indicated as a transition state with all larger basis sets the cis conformer is the only stable form. The predicted energy difference from these calculations between the cis form and the higher energy trans conformer is about 125 cm(-1) which represents essentially the barrier to internal rotation of the NCS group (rotation around NC axis). Density functional theory calculation by the B3LYP method with the same basis sets predicts this barrier to be about 25 cm(-1). By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(NC)=1.196(5), r(CS)=1.579(5), r(CN)=1.439(5), r(CC)=1.519(5)A for the distances and angles of angleCCN=112.1(5), angleCNC=146.2(5), angleNCS=174.0(5) degrees . The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.     

Rotational spectrum of a chiral alpha-hydroxyester: conformation stability and internal rotation barrier heights of methyl lactate

High resolution spectrum of methyl lactate, a chiral alpha-hydroxyester, has been investigated using a molecular jet Fourier transform microwave spectrometer. High level ab initio calculations were employed to study the conformational isomerism of methyl lactate. The observed rotational spectrum confirms that the most stable conformer has an intramolecular hydrogen bond of OH...O==C type, as predicted by the ab initio calculations. The internal rotation barrier heights of the ester methyl group and the alpha-carbon methyl group were calculated to be 5.4 and 14.5 kJ mol(-1) at the MP2/aug-cc-pVDZ level of theory for the most stable conformer. The internal rotation splittings due to the ester methyl group were observed and analyzed and the ester methyl group tunneling barrier height was determined experimentally to be 4.762 (3) kJ mol(-1).     

Conformations of alpha-aminobutyric acid in the gas phase.

A laser-ablation molecular-beam Fourier transform microwave (LA-MB-FTMW) spectrometer has been successfully applied to the structural study of alpha-aminobutyric acid. Three neutral conformers have been identified in the gas phase by comparing their experimental rotational and 14N nuclear quadrupole coupling parameters with those predicted by ab initio calculations at the MP2/6-311++G(d,p) level. The most stable conformer is stabilized by a bifurcated amine-to-carbonyl hydrogen bond (N--HO=C) and a cis-COOH group, and the side-chain adopts a configuration with a torsion angle tau(C(gamma)-C(beta)-C(alpha)-C') of about 180 degrees. The second most stable conformer exhibits the same configuration for the amino acid skeleton but adopts a different orientation for the side chain with tau(C(gamma)-C(beta)-C(alpha)-C') approximately -60 degrees. In the third conformer an intramolecular hydrogen bond is established between the hydroxyl group and the nitrogen atom (NH--O), with a side-chain orientation similar to that of the most stable conformer.     

An ab initio potential energy surface for the υ4 vibrational mode of hydrogen peroxide

In this paper, the levels and the torsional microstates of hydrogen peroxide are determined from fully optimized ab initio calculations using a nuclear model in one dimension. Calculations have been performed at the MP2 level with the 6-311 G(2df,2pd), 6-31 1+G(2df,2pd), cc-pVTZ and AUG-cc-pVTZ basis sets including polarization orbitals and diffuse functions. The most stable conformation, calculated with the MP2/AUG-cc-pVTZ approach, is a trans–gauche conformer lying at 67.5° from the trans structure. By using the same level of calculations, the heights of the trans and cis barriers have been determined to be 386.5 and 2643.8 cm⁻¹ in a good agreement with the experimental data. The variational torsional levels split into four components by the tunnelling effect of the barriers. The splitting of the fundamental level caused by the trans barrier has been found to be 11.8683 cm⁻¹, whereas the splitting caused by the cis barrier is insignificant under n=2. Current ab initio energies confirm the experimental assignments and verify the separability of the torsion from the rest of the vibrations. However, the experimental relation of dependence on the torsion of the rotational constants cannot be reproduced in one-dimension and depends on several additional vibrational effects.     

Rotational spectrum and conformational composition of cyanoacetaldehyde, a compound of potential prebiotic and astrochemical interest

The rotational spectrum of cyanoacetaldehyde (NCCH(2)CHO) has been investigated in the 19.5-80.5 and 150-500 GHz spectral regions. It is found that cyanoacetaldehyde is strongly preferred over its tautomer cyanovinylalcohol (NCCH═CHOH) in the gas phase. The spectra of two rotameric forms of cyanoacetaldehyde produced by rotation about the central C-C bond have been assigned. The C-C-C-O dihedral angle has an unusual value of 151(3)° from the synperiplanar (0°) position in one of the conformers denoted I, while this dihedral angle is exactly synperiplanar in the second rotamer called II, which therefore has C(s) symmetry. Conformer I is found to be preferred over II by 2.9(8) kJ/mol from relative intensity measurements. A double minimum potential for rotation about the central C-C bond with a small barrier maximum at the exact antiperiplanar (180°) position leads to Coriolis perturbations in the rotational spectrum of conformer I. Selected transitions of I were fitted to a Hamiltonian allowing for this sort of interaction, and the separation between the two lowest vibrational states was determined to be 58794(14) MHz [1.96112(5) cm(-1)]. Attempts to include additional transitions in the fits using this Hamiltonian failed, and it is concluded that it lacks interaction terms to account satisfactorily for all the observed transitions. The situation was different for II. More than 2000 transitions were assigned and fitted to the usual Watson Hamiltonian, which allowed very accurate values to be determined not only for the rotational constants, but for many centrifugal distortion constants as well. Two vibrationally excited states were also assigned for this form. Theoretical calculations were performed at the B3LYP, MP2, and CCSD levels of theory using large basis sets to augment the experimental work. The predictions of these calculations turned out to be in good agreement with most experimental results.     

Structural and conformational properties of fluoroformic acid anhydride, FC(O)OC(O)F

The conformational properties and geometric structures of fluoroformic acid anhydride, FC(O)OC(O)F, have been studied by vibrational spectroscopy, gas electron diffraction (GED), single-crystal X-ray diffraction, and quantum chemical calculations (HF, MP2, and B3LYP methods with 6-31G* and B3LYP/6-311+G* basis sets). Satellite bands in the IR matrix spectra, which increase in intensity when the matrix gas mixture is heated prior to deposition as a matrix, indicate the presence of two conformers at room temperature. According to the electron diffraction analysis, the prevailing conformer is of C(2) symmetry with both C=O bonds synperiplanar with respect to the opposite C-O bond ([sp, sp] conformer). The minor conformer [15(5)% from IR matrix and 6(11)% from GED] is predicted by quantum chemical calculations to possess an [sp, ac] structure. FC(O)OC(O)F crystallizes in the orthorhombic system in the space group P2(1)2(1)2(1) with a = 6.527(1) angstroms, b = 7.027(1) angstroms, and c = 16.191(1) angstroms and four formula units per unit cell. In the crystal, only the [sp, sp] conformer is present, and the structural parameters are very similar to those determined by GED.     

Rotational spectrum of 1,1,1-trifluoro-2-butanone using chirped-pulse Fourier transform microwave spectroscopy

The pure rotational spectra of 1,1,1-trifluoro-2-butanone and its four (13)C isotopologues have been studied using the new chirped-pulsed Fourier transform microwave spectrometer at the University of Manitoba in combination with a conventional Balle-Flygare-type instrument. Quantum chemical calculations, at the MP2/6-311++G(d,p) level, were carried out to obtain information about the structure, relative stability, and difference in populations of the three lowest energy conformers corresponding to dihedral angles of 0°, 82.8°, and 119.2° along the carbon backbone. The observed spectra are that of conformer I (dihedral angle 0°), and, based on analysis of the observed splitting, the V(3) barrier to internal rotation of the methyl group has been determined to be 9.380(5) kJ mol(-1). The spectroscopic constants of the five isotopologues were used to precisely derive the r(s) and partial r(0) geometries of this conformer based on an assumed planar carbon backbone (as supported by the spectra and ab initio calculations).     

Microwave, infrared and Raman spectra, r0 structural parameters, ab initio calculations and vibrational assignment of 1-fluoro-1-silacyclopentane

The microwave spectrum (6500-18 ,500 MHz) of 1-fluoro-1-silacyclopentane, c-C(4)H(8)SiHF has been recorded and 87 transitions for the (28)Si, (29)Si, (30)Si, and (13)C isotopomers have been assigned for a single conformer. Infrared spectra (3050-350 cm(-1)) of the gas and solid and Raman spectrum (3100-40 cm(-1)) of the liquid have also been recorded. The vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twist form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but much lower energy than the planar conformer. By utilizing the microwave rotational constants for seven isotopomers ((28)Si, (29)Si, (30)Si, and four (13)C) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the twist conformer. The heavy atom distances in A? are: r(0)(SiC(2)) = 1.875(3); r(0)(SiC(3)) = 1.872(3); r(0)(C(2)C(4)) = 1.549(3); r(0)(C(3)C(5)) = 1.547(3); r(0)(C(4)C(5)) = 1.542(3); r(0)(SiF) = 1.598(3) and the angles in degrees are: [angle]CSiC = 96.7(5); [angle]SiC(2)C(4) = 103.6(5); [angle]SiC(3)C(5) = 102.9(5); [angle]C(2)C(4)C(5) = 108.4(5); [angle]C(3)C(5)C(4) = 108.1(5); [angle]F(6)Si(1)C(2) = 110.7(5); [angle]F(6)Si(1)C(3) = 111.6(5). The heavy atom ring parameters are compared to the corresponding r(s) parameters. Normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values, and infrared band contours. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.     

Structural studies on ethyl isovalerate by microwave spectroscopy and quantum chemical calculations

We observed the microwave spectrum of ethyl isovalerate by molecular beam Fourier transform microwave spectroscopy. The rotational and centrifugal distortion constants of the most abundant conformer were determined. Its structure was investigated by comparison of the experimental rotational constants with those obtained by ab initio methods. In a first step, the rotational constants of various conformers were calculated at the MP2/6-311++G** level of theory. Surprisingly, no agreement with the experimental results was found. Therefore, we concluded that in the case of ethyl isovalerate more advanced quantum chemical methods are required to obtain a reliable molecular geometry. Ab initio calculations carried out at MP3/6-311++G**, MP4/6-311++G**, and CCSD/6-311++G** levels and also density functional theory calculations using the B3LYP/6-311++G** method gave similar results for the rotational constants, but they were clearly distinct from those obtained at the MP2/6-311++G** level. With use of these more advanced methods, the rotational constants of the lowest energy conformer were in good agreement with those obtained from the microwave spectrum.     

Equatorial and axial hydrogens in heterocyclic six-membered rings: the rotational spectrum of piperazine

The rotational spectrum of piperazine has been investigated by free jet absorption millimeter wave spectroscopy. The spectrum of the polar conformer with axial–equatorial orientations of the two amino hydrogens was only observed. This assignment was confirmed by the spectra of the two monodeuterated and the bideuterated species.Ab initio and density functional calculations predict the observed conformer to have an energy intermediate between the equatorial–equatorial and axial–axial non-polar forms, the former species being the global minimum.     

A microwave and quantum chemical study of the conformational properties and intramolecular hydrogen bonding of 1-fluorocyclopropanecarboxylic acid

The structural and conformational properties of 1-fluorocyclopropanecarboxylic acid have been explored by microwave spectroscopy and a series of ab initio (MP2/6-311++G(d,p) level), density functional theory (B3LYP/aug-cc-pVTZ level), and G3 quantum chemical calculations. Four "stable" conformers, denoted conformers I-IV, were found in the quantum chemical calculations, three of which (conformers I -III) were predicted to be low-energy forms. Conformer I was in all the quantum chemical calculations predicted to have the lowest energy, conformer III to have the second lowest energy, and conformer II to have the third lowest energy. Conformers II and III were calculated to have relatively large dipole moments, while conformer I was predicted to have a small dipole moment. The microwave spectrum was investigated in the 18-62 GHz spectral range. The microwave spectra of conformers II and III were assigned. Conformer I was not assigned presumably because its dipole moment is comparatively small. Conformer II is stabilized by an intramolecular hydrogen bond formed between the fluorine atom and the hydrogen atom of the carboxylic acid group. Conformer III has a synperiplanar orientation for the F-C-C=O and H-O-C=O chains of atoms. Its dipole moment is: mua = 3.4(10), mub = 10.1(13), and muc = 0.0 (assumed) and mu(tot) = 10.6(14) x 10(-30) C m [3.2(4) D]. Several vibrationally excited states of the lowest torsional mode of each of II and III were also assigned. The hydrogen-bonded conformer II was found to be 2.7(2) kJ/mol less stable than III by relative intensity measurements. Absolute intensity measurements were used to show that the unassigned conformer I is the most abundant form present at a concentration of roughly 65% at room temperature. Conformer I was estimated to be ca. 5.0 kJ/mol more stable than the hydrogen-bonded rotamer (conformer II) and ca. 2.3 kJ/mol more stable than conformer III. The best agreement with the theoretical calculations is found in the MP2 calculations, which predict conformer I to be 5.1 kJ/mol more stable than III and 1.7 kJ/mol more stable than II.