Mirjana Mladenović

Rovibrational Hamiltonians for general polyatomic molecules in spherical polar parametrization. I. Orthogonal representations

The interdependence of the description of the internal geometry and the corresponding kinetic energy operator T is investigated in detail for a general n-atomic molecule. For both space-fixed and body-fixed reference frames compact expressions of T are derived which are applicable to any set of n−1 translationally and rotationally invariant internal vectors in a spherical polar parametrization. Simple analytical forms are given for reduced masses and kinetic coupling constants, which are the only vector specific parameters in the final rovibrational kinetic energy expression. The kinetic energy assumes the most separable form for an entirely orthogonal set of internal vectors. A highly efficient computer program for the calculation of rovibrational spectra of tetratomic molecules has been developed on the basis of this formulation. Calculations on the HF dimer and the metastable molecule HOCO illustrate the accuracy and flexibility of this approach.

Theoretical study of the rovibrational energy spectrum and the numbers and densities of bound vibrational states for the system HCO+/HOC+

Large-scale calculations of accurate energy levels for the system HCO+/HOC+ in its electronic ground state are reported. The rovibrational levels are calculated for total angular momentum J=0, 1, and 2 by means of the discrete variable representation of the angular coordinate in conjunction with a distributed Gaussian basis for the radial degrees of freedom. A new analytical potential energy surface is used which is based on high level ab initio calculations [CCSD(T)/cc-pVQZ]. The rovibrational energy spectra of HCO+ and HOC+, as well as of the isotopomers DCO+ and DOC+, are analyzed in detail up to the ground state adiabatic isomerization barrier at 28 798 cm−1. Spectroscopic parameters calculated for low lying vibrational states show distinct differences between HCO+ and HOC+. A total number of 6042 bound states up to the first classical dissociation limit (H++CO) at 51 621 cm−1 and a density at the threshold of 0.52/cm−1 are estimated for J=0. Semiclassical phase space integration yields nearly identic...

Rovibrational Hamiltonians for general polyatomic molecules in spherical polar parametrization. II. Nonorthogonal descriptions of internal molecular geometry

A rovibrational kinetic energy operator for a two-vector embedded reference frame is derived for general polyatomic molecules of arbitrary structure. New compact expressions for T in terms of angular momentum and linear momentum vector operators are explicitly given for tri-, tetra-, and pentatomic molecules. The necessary actions for the evaluation of kinetic energy matrix elements in a standard rotation-angular basis are summarized. The structure of kinetic energy operators in the body-fixed formulation is explained with the help of Cartesian components of angular momentum and linear momentum vector operators. Kinetic energy operators for tetratomic molecules in a bisector embedded body-fixed formulation and for sequentially bonded pentatomic molecules are derived to show how to use our general result for diverse coordinate systems.

Isofulminic acid, HONC: Ab initio theory and microwave spectroscopy

Isofulminic acid, HONC, the most energetic stable isomer of isocyanic acid HNCO, higher in energy by 84 kcal/mol, has been detected spectroscopically by rotational spectroscopy supported by coupled cluster electronic structure calculations. The fundamental rotational transitions of the normal, carbon-13, oxygen-18, and deuterium isotopic species have been detected in the centimeter band in a molecular beam by Fourier transform microwave spectroscopy, and rotational constants and nitrogen and deuterium quadrupole coupling constants have been derived. The measured constants agree well with those predicted by ab initio calculations. A number of other electronic and spectroscopic parameters of isofulminic acid, including the dipole moment, vibrational frequencies, infrared intensities, and centrifugal distortion constants have been calculated at a high level of theory. Isofulminic acid is a good candidate for astronomical detection with radio telescopes because it is highly polar and its more stable isomers (HNCO, HOCN, and HCNO) have all been identified in space.

Ion-molecule reactions involving HCO+ and N2H+: Isotopologue equilibria from new theoretical calculations and consequences for interstellar isotope fractionation

Aims. We revisit with new augmented accuracy the theoretical dynamics of basic isotope exchange reactions involved in the 12 C/ 13 C, 16 O/ 18 O, and 14 N/ 15 N balance because these reactions have already been studied experimentally in great detail. Methods. Electronic structure methods were employed to explore potential energy surfaces, full-dimensional rovibrational calculations to compute rovibrational energy levels that are numerically exact, and chemical network models to estimate the abundance ratios under interstellar conditions. Results. New exothermicities, derived for HCO + reacting with CO, provide rate coefficients markedly different from previous theoretical values in particular at low temperatures, resulting in new abundance ratios relevant for carbon chemistry networks. In concrete terms, we obtain a reduction in the abundance of H 12 C 18 O + and an increase in the abundance of H 13 C 16 O + and D 13 C 16 O + .I n all studied cases, the reaction of the ion with a neutral polarizable molecule proceeds through the intermediate proton-bound complex found to be very stable. For the complexes OCH + ··· CO, OCH + ·· ·OC, COHOC + ,N 2 ··· HCO + ,N 2H + ··· OC, and N2HN + ,w e also calculated vibrational frequencies and dissociation energies. Conclusions. The linear proton-bound complexes possess sizeable dipole moments, which may facilitate their detection.

Vibrational calculation for the HOCO radical and the cis-HOCO anion

We present numerically exact vibrational transitions for trans-HOCO, cis-HOCO, and cis-HOCO(-) for the quartic force fields of Fortenberry et al. [J. Chem. Phys. 135, 134301 (2011); ibid. 135, 214303 (2011)], obtained by means of a computational strategy based on the discrete variable representation. Several adiabatic projection schemes have been employed to characterize the vibrational levels and to study the relevance of the intermode coupling (vibrational mixing). Our results help to clear up a large discrepancy between previously reported vibrational perturbation theory and vibrational configuration interaction predictions for the torsional frequency.

Intermolecular interaction in an open-shell π-bound cationic complex: IR spectrum and coupled cluster calculations for C2H2+-Ar

The intermolecular potential energy surface (PES) of Ar interacting with the acetylene cation in its (2)Pi(u) ground electronic state is characterized by infrared photodissociation (IRPD) spectroscopy and quantum chemical calculations. In agreement with the theoretical predictions, the rovibrational analysis of the IRPD spectrum of C(2)H(2) (+)-Ar recorded in the vicinity of the antisymmetric CH stretching fundamental (nu(3)) is consistent with a vibrationally averaged T-shaped structure and a ground-state center-of-mass separation of R(c.m.) = 2.86 +/- 0.09 A. The nu(3) band experiences a blueshift of 16.7 cm(-1) upon complexation, indicating that vibrational excitation slightly reduces the interaction strength. The two-dimensional intermolecular PES of C(2)H(2) (+)-Ar, obtained from coupled cluster calculations with a large basis set, features strong angular-radial coupling and supports in addition to a global pi-bound minimum also two shallow side wells with linear H-bound geometries. Bound state rovibrational energy level calculations are carried out for rotational angular momentum J = 0-10 (both parities) employing a discrete variable representation-distributed Gaussian basis method. Effective spectroscopic constants are determined for the vibrational ground state by fitting the calculated rotational energies to the standard Watson A-type Hamiltonian for a slightly asymmetric prolate top.

Rovibrational Hamiltonians for general polyatomic molecules in spherical polar parametrization. III. Global vs local axis system and angular coordinates

We study different parametrizations of the angular space of polyatomic molecules for an orthogonal description of the molecular geometry. Kinetic energy operators for pentatomic molecules, given by compact and computationally useful forms in a global and a local formulation of the axis system, are compared and discussed. A new decomposition of T for sequentially bonded pentatomic molecules in conjunction with a basis of Wigner and associated Legendre functions provides kinetic energy matrix elements which are free of singularities. Practical problems caused by an unusual volume element for a description involving only intervector (bending) angles are addressed. The corresponding rovibrational kinetic energy operators are derived for the two-vector body-fixed formulation.

Quasilinearity in tetratomic molecules: An ab initio study of the CHNO family

We present coupled-cluster CCSD(T) all electron results for the equilibrium structure of isofulminic acid, HONC, together with results for the barrier to linearity and the energetics for the four most stable members of the CHNO isomer family, obtained for the ground electronic states by means of large correlation consistent basis sets. Minimum energy paths along the angular coordinates reported for these CHNO isomers are combined with the dominant kinetic energy contributions to predict key rovibrational spectroscopic features which are clearly reminiscent of quasilinear behavior in tetratomic molecules.

Electric and magnetic properties of the four most stable CHNO isomers from ab initio CCSD(T) studies

Electric and magnetic properties obtained from CCSD(T)/(aug-)cc-pCVXZ (X = T, Q, or 5) electronic structure calculations are reported for isocyanic acid, HNCO, cyanic acid, HOCN, fulminic acid, HCNO, and isofulminic acid, HONC, in their ground electronic states. Comparison of the theoretical results with the available experimentally derived values shows very satisfactory agreement. The new data should be helpful for the identification of these molecules due to characteristic hyperfine structure patterns in their microwave spectra. A brief discussion of the electronic structure properties, based on the electric field gradients, Mulliken population analysis of the total electron density, and molecular orbitals, is provided for the four CHNO isomers and the related HCN/HNC system.