Search

A highly accurate CO

Protonated acetylene, C2H3(+), is among the simplest carbocations. Comprehensive experimental or highly accurate computational spectroscopic data is lacking for this system due to its inherent complexities. Utilizing state-of-the-art quartic force fields (QFFs), the spectroscopic constants and fundamental vibrational frequencies are provided in this work for the nonclassical, bridged, cyclic global minimum. The rotational constants match experiment to better than 0.1%, and the computed ν2 antisymmetric HCCH stretch is less than 3.0 cm(–1) different from experiment. Hence, the rovibrational spectroscopic data provided herein for c-C2H3(+) and its deuterated isotopologues enrich the chemical understanding of this system. Unfortunately, the same rovibrational spectroscopic data is not as trustworthy for the classical, linear form of protonated acetylene due to the shallow well in which it resides on the potential energy surface. However, spectroscopic data are provided for this isomer in the Supporting Information to enhance future studies.

A new approach is proposed and investigated for approximately including the effects of diffuse functions in one-particle basis sets when high accuracy is desired. The method is cost-effective for use in computing quartic force fields (QFFs), global potential energy surfaces (PESs), or other situations when a large part of the PES is needed. It is conservatively estimated that the use of this approximation leads to a computational savings of a factor of five, and it is argued that this could be significantly larger if input/output wait times are considered. It can be used when extrapolation to the one-particle basis set limit is performed, or it can be used simply to approximate the effect of diffuse functions for a larger basis set. The new approach is based on scaling the diffuse function effect for a smaller basis set to approximate the effect for a larger basis or an extrapolated energy in which larger basis set(s) are used. The scale factor is written as a function of the geometrical coordinates of the molecule and thus it includes a geometry dependence. We report results where the scale factor is a constant, includes through gradient terms, includes through second derivative terms, and includes through diagonal second and third derivative terms. The method has been tested in the calculation of accurate QFFs, equilibrium structures, and harmonic and fundamental vibrational frequencies for NH(2)(-), OH(-), H(2)O, and CH(3)OH. It is found that including up through diagonal second derivative terms leads to reliable fundamental vibrational frequencies and is cost-effective. It is also concluded that the use of a 5Z-quality basis set is essential if high accuracy is desired for these properties, even with extrapolation to the one-particle basis set limit.

An accurate potential energy surface has been determined for methanol from ab initio potential data at the CCSD(T) level of theory with an aug-cc-pVTZ basis. The resulting potential function is valid over all twelve vibrational degrees of freedom for all near-equilibrium and torsional configurations. A torsional reaction path has been derived for this potential, from which the low-lying vibrational levels of methanol have been calculated by the reaction path version of MULTIMODE. Comparisons with experiment and other calculations are made.

We present argon predissociation vibrational spectra of the OH(-).H(2)O and Cl(-).H(2)O complexes in the 1000-1900 cm(-1) energy range, far below the OH stretching region reported in previous studies. This extension allows us to explore the fundamental transitions of the intramolecular bending vibrations associated with the water molecule, as well as that of the shared proton inferred from previous assignments of overtones in the higher energy region. Although the water bending fundamental in the Cl(-).H(2)O spectrum is in very good agreement with expectations, the OH(-).H(2)O spectrum is quite different than anticipated, being dominated by a strong feature at 1090 cm(-1). New full-dimensionality calculations of the OH(-).H(2)O vibrational level structure using diffusion Monte Carlo and the VSCF/CI methods indicate this band arises from excitation of the shared proton.