Ground and first excited torsional states of acetamide (original) (raw)
Related papers
Journal of Molecular Spectroscopy, 2008
Stimulated by recent THz measurements of the methanol spectrum in one of our laboratories, undertaken in support of NASA programs related to the Herschel Space Observatory (HSO) and the Atacama Large Millimeter Array (ALMA), we have carried out a global analysis of available microwave and high-resolution infrared data for the first three torsional states (m t = 0, 1, 2), and for J values up to 30. This global fit of approximately 5600 frequency measurements and 19 000 Fourier transform far infrared (FTFIR) wavenumber measurements to 119 parameters reaches the estimated experimental measurement accuracy for the FTFIR transitions, and about twice the estimated experimental measurement accuracy for the microwave, submillimeter-wave, and terahertz transitions. The present fit is essentially a continuation of our earlier work, but we have greatly expanded our previous data set and have added a large number of new torsion-rotation interaction terms to the Hamiltonian in our previously used computer program. The results, together with a number of calculated (but unmeasured) transitions, including their line strength, estimated uncertainty, and lower state energy, are made available in the supplementary material as a database formatted to be useful for astronomical searches. Some discussion of several open spectroscopic problems, e.g., (i) an improved notation for the numerous parameters in the torsion-rotation Hamiltonian, (ii) possible causes of the failure to fit frequency measurements to the estimated measurement uncertainty, and (iii) pitfalls to be avoided when intercomparing apparently identical parameters from the internal axis method and the rho axis method are also given.
Avoided-crossing molecular-beam study of the torsion-rotation energy levels of CH3CF3
Chemical Physics, 1991
The avoided-crossing molecular-beam electric resonance method has been used to determine the leading parameters in the torsion-rotation Hamiltonian of CH&F,, which was selected as a prototype for the case of a symmetric rotor with small ( Q 500 kHz) internal rotor splittings. Stark anticrossings have been studied for (K= f 2-r 1) with J= 2-6; hypertine anticrossings have been studied for (K= &2-O), ( f2*k 1). and ( z!z l-0) with J<2. A detailed investigation of the hyperfine case has been carried out involving selection rules, relative intensity calculations, Zeeman studies, and combination differences. Several pure rotational transitions for the lowest two torsional states have been measured between 93 and 114 GHz with a mm-wave spectrometer. The (J= 1+-O) transition in the ground torsional state has been measured with the molecular beam spectrometer. Stark and Zeeman studies have been carried out with conventional molecular beam techniques. It has been determined that the effective rotational constant Afl= 5502.904( 3) MHz, the effective height ofthe threefold barrier to internal rotation VH= 1093 ( 11) cm-' and the moment of inertia of the methyl top I,= 3.2 l(4) amu A2. It has been found that the electric dipole moment ,u= 2.34720( 13) D and the distortion dipole moment constant pr,= 3.220( 11) pD. The magnitudes and signs of the molecularg-factors have been obtained: g, = -0.0226 ( 13 ) nm and g, = -0.117 ( 1) nm. In addition, values have been determined for the B-rotational constant and several distortion constants.
Journal of Molecular Spectroscopy, 2004
The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH 3 ANHAC(@O)ACH 3 ), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G 18 (not isomorphic to any point group), with irreducible representations A 1 , A 2 , E 1 , E 2 , E 3 , and E 4 . One of these symmetry species and the usual three asymmetric rotor quantum numbers J KaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where F ¼ J þ I N . Transitions involving levels of A 1 and A 2 species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion-rotation transitions involving 152 torsion-rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles w and / often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm À1 , respectively. Top-top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule.
Journal of Molecular Spectroscopy, 2010
The aim of the present paper is to investigate the use of quantum chemistry calculations to obtain the torsional dependence of various structural and vibrational-force-field-related quantities that could help in estimating the vibration-torsion-rotation interaction terms needed to treat perturbations observed in the spectra of methanol-like molecules. We begin by using the Gaussian suite of programs to determine the steepest-descent path from a stationary point at the top of the internal rotation potential barrier in methanol to the equilibrium structure at the bottom of the barrier. This procedure requires determining the gradient Vofthepotential(ascalculatedinmass−weightedCartesiancoordinates)alongtheinternalrotationpath.Inaddition,weusetheGaussiansuitetocalculatetheHessianV of the potential (as calculated in mass-weighted Cartesian coordinates) along the internal rotation path. In addition, we use the Gaussian suite to calculate the Hessian Vofthepotential(ascalculatedinmass−weightedCartesiancoordinates)alongtheinternalrotationpath.Inaddition,weusetheGaussiansuitetocalculatetheHessian$V along this path and to generate from these second derivatives the 3N À 7 small-amplitude vibrational frequencies and the 3N Cartesian vibrational displacements for each of these vibrations. We then symmetrize the internal coordinates used in presenting the structures, gradients, Hessians and vibrational displacements along the path to take into account the periodic variation of the behavior of the three methyl hydrogen atoms H i as they pass in turn through the C s -plane of the HOC frame. The symmetrized linear combinations of the CH i stretches, of the OCH i bends, and of the HOCH i dihedral angles of the methyl group depend on the internal rotation angle c and they are determined by considering coordinate transformations from the G 6 permutation-inversion group appropriate for internally rotating methanol. This symmetrization procedure permits us to explore the feasibility of expressing the structures, gradients, Hessians, and vibrational displacement vectors along the internal rotation path as short Fourier series in c, which is one of the main goals of this paper. In summary, we find that the symmetrized structures, gradients, and Hessians, as well as nine of the 11 projected vibrational frequencies and the vibrational displacement vectors for the three vibrations occurring primarily in the HOC frame can be expressed by short Fourier series expansions to their precision in the Gaussian output, and that these series involve only sin 3nc or only cos 3nc terms, as required by G 6 symmetry considerations. A preliminary discussion is given of why short Fourier expansions fail for the projected frequencies of the two methyl asymmetric stretches, and for the vibrational displacement vectors of the methyl group vibrational modes. Looking more closely at the symmetrized and projected 3N Â 3N Hessian, we find algebraically that only elements in the (3N À 7) Â (3N À 7) small-amplitude-vibrational block of the Hessian are useful for spectroscopic problems. Non-zero elements in the rest of the 3N Â 3N symmetrized and projected Hessian cannot be converted into quantities needed for perturbation studies.
A new joint analysis of the ground and first excited torsional states of methylformate
Journal of Molecular Spectroscopy, 2009
A global fit within experimental accuracy of microwave rotational transitions in the ground and first excited torsional states (v t = 0 and 1) of methylformate (HCOOCH 3) is reported, which combines older measurements from the literature with new measurements from Kharkov. In this study the so-called ''rho axis method'' that treats simultaneously both A and E species of the ground and first excited torsional states is used. The final fit requires 55 parameters to achieve an overall unitless weighted standard deviation of 0.71 for a total of 10533 transitions (corresponding to 9298 measured lines) with rotational quantum numbers up to J 6 62 and K a 6 26 in the ground state and J 6 35 and K a 6 23 in the first excited torsional state. These results represent a significant improvement over past fitting attempts, providing for the first time a fit within experimental accuracy of both ground and first excited torsional states.
Journal of Molecular Structure, 2004
An advanced graphically oriented interactive program package for assignments of complex (perturbed) vibration-rotation spectra of asymmetric and symmetric top molecules has been developed. In addition to the well known Loomis-Wood algorithm, the new procedure takes advantage of a precise knowledge of the lower (e.g. ground) vibrational state energies, works with a realistic approximation of effective Hamiltonians for lower as well as upper vibrational states, and allows an instant combination difference inspection of spectral lines by the graphical representation of the appropriate parts of the analyzed experimental spectrum. Being constrained to the combination difference checking, the new algorithm can directly assign the correct rotational quantum numbers as well as 'quality weights' estimating relative accuracies of the identified lines.
One-dimensional models of internal rotation in CX 3NO molecules (X = H, D, F
Journal of Molecular Spectroscopy, 2005
Various non-empirical methods for estimating the parameters of one-dimensional internal rotation potentials and energies of torsional transitions were compared for the CX 3 NO molecules (X = H, D, F) in the ground (S 0 ) and lowest excited singlet (S 1 ) electronic states. The potential energy surfaces were studied by the ab initio MR-AQCC/cc-pVTZ, MR-AQCC/cc-pVTZ(-f), MP2/6-311G(2d), and MP2/6-311G(d,p) methods. The one-dimensional internal rotation problem was solved using the following models: (1) geometry optimization at a given internal rotation coordinate; (2) intrinsic reaction path; (3) gradient extremal; and (4) use of only the data on potential energy surface stationary points. Special attention was paid to the problem of calculation of kinematic coefficient. In all cases, the calculated torsional energies for CX 3 NO molecules (X = H, D, F) are in agreement with experiment. The results from different methods for constructing torsional cross-sections of the potential energy surface are virtually equivalent and differ insignificantly from the results of calculations within the framework of the simplest model, hence, estimates of the barrier to internal rotation are of most importance. It was found that a change in the zero-point energy could give a correction to the internal rotation potential as large as 15% of the potential barrier. However, in the case under consideration the calculations in the harmonic approximation taking into account this correction do not improve the agreement between the calculated torsional transitions and the experimental data.
A general treatment of vibration-rotation coordinates for triatomic molecules
International Journal of Quantum Chemistry, 1991
An exact, within the Born-Oppenheimer approximation, body-fixed Hamiltonian for the nuclear motions of a triatomic system is presented. This Hamiltonian is expressed in terms of two arbitrarily defined internal distances and the angle between them. The body-fixed axis system is related to these coordinates in a general fashion. Problems with singularities and the domain of the Hamiltonian are discussed using specific examples of axis embedding. A number of commonly used coordinate systems including Jacobi, bond-length-bond-angle, and Radau coordinates are special cases of this Hamiltonian. Sample calculations on the HzS molecule are presented using all these and other coordinate systems. The possibility of using this Hamiltonian for reactive scattering calculations is also discussed.
Computer Physics Communications, 1988
The results of some highly accurate non-empirical ro-vibrational calculations on H 2D~are reported including some for J 30 which are the first calculations to describe such a highly rotationally excited state. These results are obtained using an improved version of our algorithm. The method used is a variational one and is well adapted to execution on supercomputers. The computational characteristics of the performance the method on the CRAY Is and the CRAY XMP 48 are given.