Hs.H. Günthard

Methanol and deuterated species: Infrared data, valence force field, rotamers, and conformation☆

Abstract New infrared measurements of gaseous and matrix-isolated methanol and 7 deuterated species are presented and analyzed. A revised assignment for the species CH 3 OH(D) and CD 3 OH(D) is used to determine 15 significant parameters of a valence force field by a converged simultaneous least-squares adjustment to 44 observed fundamentals. The respective frequencies calculated for the rotamers of CH 2 DOH(D) and CHD 2 OH(D) turn out to be a valid prediction as they are used to assign 78 observed fundamentals of the species with partially deuterated methyl groups. The values of the 15 force constant parameters are refined by including all 122 observed fundamentals. The force field contains significant deviations from local C 3 v symmetry of the methyl group. Such an asymmetry, in the CH-stretching diagonal part, produces a difference of about 10 cm −1 in the zero point energy of symmetric and asymmetric rotamers of the species with partially deuterated methyl groups. Calculations based on equilibrium structures with and without methyl tilt yield better agreement with observed data in the nontilted case. A preliminary calculation of relative intensities reproduces the major effects of isotopic substitution and rotational isomerism. Experimental evidence for the staggered conformation is obtained from a comparison of observed data with calculated results based on staggered and eclipsed models.

Infrared Membrane Spectroscopy

Application of vibrational spectroscopy to the problem of structure determination of molecules of biological interest goes back to the early uses of raman and infrared spectroscopy in the study of organic molecules. For reviews of earlier work the reader is referred to compilations by Kohlrausch (1943) and by Jones and Sandorfy (1956), whereas more recently a comprehensive discussion has been presented by Bellamy (1975). These compilations accentuate the correlation of vibrational spectra with molecular structure from an essentially empirical point of view and culminate in the establishment of empirical correlation charts. For typical examples the reader is referred to Weast (1974) and Bellamy (1975). There have been many treatments of the theoretical basis of molecular vibrational spectroscopy. Among them the classical work by Herzberg (1945) and by Wilson et al. (1955) should be mentioned. Applications of infrared spectroscopy (IR) to structure problems of biological interest have been summarized by Susi (1969), Fraser and MacRae (1973), and Wallach and Winzler (1974). It was remarked quite eraly that relevant structural information about biological systems often requires study in aqueous solution, which forms the natural environment for most biologically important systems. Besides critical control of experimental conditions and samples the conventional methods of raman spectroscopy may be applied to aqueous solutions in a quite straightforward manner, cf. the contribution by Lord and Mendelson, Chapter 8.

General Internal Motion of Molecules, Classical and Quantum‐Mechanical Hamiltonian

The dynamics and the Hamiltonian of a general asymmetric‐top molecule undergoing almost arbitrary deformations are discussed. s vector notations are used for translational, rotational, and internal‐velocity coordinates. The kinetic energy is formulated by generalizing the G‐matrix technique known from the theory of molecular vibrations. A geometrical definition of the rotational coordinates referring to the instantaneous principal axis system is compared with a dynamical definition involving the over‐all angular momentum. States of general internal motion are associated by definition with zero linear momentum and zero over‐all angular momentum.

A versatile method for molecular structure determinations from ground state rotational constants

Abstract A new procedure is described to determine the geometrical structure of a molecule. It starts from measured ground state rotational constants of isotopically substituted species. Internal structural parameters such as bond lengths, bond angles and dihedral angles are directly fitted with a suitable least-squares algorithm. The new method for structure determination is compared to the usual Costain—Kraitchman substitution method. It contains less stringent conditions than the latter, permits a broad range of applications and provides a reliable molecular structure.

Rotational isomerism in methyl formate and methyl acetate; a low-temperature matrix infrared study using thermal molecular beams

Abstract IR spectra of methyl formate and methyl acetate have been studied by trapping thermal molecular beams in Ar matrices. For methyl formate, four bands have been assigned to the anti conformer. Δ H o (anti-syn) for the conformational equilibrium is 4750 ± 190 cal mol −1 . In methyl acetate there is an increase in Δ H o due to steric hindrance between the methyl groups.

Internal rotation in acetaldehyde

Abstract The microwave spectrum of acetaldehyde has been investigated in the frequency range from 7 to 40 GHz. A rather complete assignment of rotational transitions in the ground and torsionally excited states has been found with the help of microwave-microwave double resonance techniques. The spectral data have been analyzed using three different models for the overall and internal rotation problem including a nonrigid model. The threefold component of the internal rotation barrier was determined to be V3 = 400 ± 2 cm−1. The sixfold contribution V6 = −10.9 ± 0.3 cm−1 could only be adjusted reliably from data for both ground and torsionally excited states using the nonrigid model. The methods of barrier determinations have been critically reviewed. In an appendix, the Hamiltonian for a nonrigid model is derived based on structure relaxation of the methyl top during internal rotation.

The microwave spectrum and dipole moment of azulene

Abstract The microwave spectrum of azulene (C 10 H 8 ) has been investigated in the frequency region 7–13 GHz. Rotational transitions for the ground state and one excited vibrational state could be assigned and the following rotational constants have been determined for the ground state: A = 2841.951 ± 0.024 MHz, B = 1254.8463 ± 0.0010 MHz, and C = 870.7162 ± 0.0008 MHz; and for the exeited state: A = 2841.73 ± 0.22 MHz, B = 1254.771 ± 0.008 MHz, and C = 871.478 ± 0.007 MHz. The Stark effect of four ground-state rotational transitions has been analyzed and the dipole moment has been determined to be 0.796 ± 0.014 D lying only in the a axis. Evidence is given for C 2 v symmetry of azulene considering Stark effects and inertial defects.

Theory of rotation and torsion spectra for a semi-rigid model of molecules with an internal rotor of C 2v symmetry

The theory of the rotation-internal rotation spectra for a rather general class of molecules with an internal rotor of the symmetry C 2v is developed in the semi-rigid model approximation. Analytical expressions for the classical and quantum-mechanical hamiltonian are given. It is shown that the energy matrix may be analytically calculated in an appropriate zeroth-order basis composed of rotational and trigonometric wave functions. Symmetry properties of the hamiltonian are given and used extensively in factoring the energy matrix. The irreducible blocks of this matrix are of infinite order. The asymptotic behaviour of the matrix elements far from the diagnoal is shown to be O(ηn ) (η < 1; n is the distance from the main diagonal), which is of decisive importance in the numerical diagonalization of such matrices. A few numerical examples for a model are given in order to demonstrate the high precision achievable in the infinite matrix diagonalization. Furthermore the internal rotation problem (J = 0 state...

A transferable valence force field for polyatomic molecules: A scheme for a series of molecules containing CO groups

Abstract The definition of a transferable valence force field is given for a series of molecules containing CO groups, including pyruvic acid, acetone, acetic acid, methyl acetate, formic acid and methyl formate. Its main characteristics might be applied to any other molecule or group of molecules. The limitation of the force fields through transferability requirements opens expectations for a strong data reduction. A set of 144 force constants is adjusted to 308 frequencies and 232 shifts of 61 isotopic modifications. The good fit achieved (rms-deviation: 6.38 and 1.26 cm−1 for frequencies and shifts, respectively) suggests the present type of constraining to be very valuable. Furthermore, systematic trends concerning magnitudes and signs of distinct types of force constants as well as good agreement with ab initio values in the case of formic acid indicate physical significance. A PED-characterization of the fundamentals of the main parent species is given. In the case of methyl acetate the calculations suggest a revision of published assignments.

Conformational interconversions in supersonic jets: Matrix IR spectroscopy and model calculations

Abstract Terminal conformer populations in supersonic molecular beams have been measured by use of matrix IR spectroscopy. the experimental technique is based on trapping of the beam molecules in to a cryogenic matrix. The ratio of conformational isomers is determined by comparing intensity ratios of infrared absorption bands with those found in analogous experiments with thermal effusive molecular beams. supersonic beams of pure 1,2-difluoroethane a considerable depopulation fo the less stable trans conformer is found, the lowest terminal conformational temperature reached being Tct8 = 207 (3) K. In seeded argon beams the cooling of the conformational distribution was found to be weaker. In supersonic beams of 1,2-dichloroethane and of methyl nitrite no significant conformational cooling was found. The experimental results are discussed in terms of a kinetic model of conformational interconversion in the flow field of a continuum free jet. The calculation indicate that conformational cooling by supersonic expansion may be expected only for molecules with a low energy barrier to internal rotation.