Simple perturbation theory estimates of equilibrium constants from force fields

Abstract

A combined quantum chemical and transition state theory study of the C2H+ 2+CH4 reaction dynamicsWe propose and test a very simple method for calculating equilibrium constants from quartic force fields.

A coupled-cluster study of the enthalpy of formation of nitrogen sulfide, NS

“…Experimental harmonic frequencies x e and anharmonic terms x e x e were used to derive zero-point vibrational energy corrections [11,20]. In principle, a further small contribution ÔE 0 Õ to the zero-point energy arises from third and fourth derivatives of the molecular potential energy [27]. We investigated this for NS at the CCSD(T)/aug-cc-pVTZ level over AE0:1 A A around the equilibrium geometry, and found E 0 to be less than 0:2 kJ mol À1 .…”

A new approach to the exact and approximate anharmonic vibrational partition function of diatomic and polyatomic molecules utilizing Morse and Rosen–Morse oscillators

“…Morse potential serves as a real accurate model for representing anharmonic vibrations in diatomic molecules and stretching modes in polyatomic molecules, and hence has received increased attention in many areas in chemistry and physics [6][7][8][9][10][11]. Furthermore, Morse potential under certain conditions may mimic Lennard-Jones (12-6) potential, which is ubiquitous in modeling intermolecular forces, and connection formula is drawn between the two potentials [12].…”

A general time-dependent route to Resonance-Raman spectroscopy including Franck-Condon, Herzberg-Teller and Duschinsky effects

“…Indeed, matrices C and D are then diagonal: (42) As a consequence, the autocorrelation function becomes, (43) As done for the general case, the polarizability tensor can be written as a function of derivatives of χ 0 with respect to the elements of the vector v. The expression is further simplified since D is diagonal, so we have, (44) As a result, the polarizability tensor at the Franck-Condon level can be written in the following way:…”

“…Those anharmonic frequencies are then used in place of the harmonic ones in the definition of the cross-correlation function, which leads to corrected intensities too. Moreover, anharmonic frequencies can be used to compute accurate partition functions and so abundances of species inside mixtures, when necessary to match experimental conditions 42,43 . Regarding the excited electronic state, if analytical force constants are not available, which is the case for most quantum chemical packages, calculation of anharmonic frequencies at the VPT2 level becomes prohibitive.…”