A purely ab initio spectroscopic quality quartic force field for acetylene

Abstract

The quartic force field of acetylene was determined using the CCSD͑T͒ method ͑coupled cluster with all single and double substitutions and quasiperturbative inclusion of connected triple excitations͒ with a variety of one-particle basis sets of the atomic natural orbital, correlation consistent, and augmented correlation consistent types. The harmonic g bending frequency 4 and the corresponding anharmonicity 4 Ϫ 4 are both found to be extremely sensitive to the basis set used, in particular to the presence of a sufficient complement of diffuse functions. ͑Due to symmetry cancellation, the corresponding effect on the u mode, i.e., 5 and 5 Ϫ 5 , is much weaker.͒ Similar phenomena are observed more generally in bending modes for molecules that possess carbon-carbon multiple bonds. Tentative explanations are advanced. Our best computed quartic force field, which combines CCSD͑T͒/͓6s5 p4d3 f 2g/4s3 p2d1 f ͔ anharmonicities with a geometry and harmonic frequencies that additionally include inner-shell correlation effects, reproduces the observed fundamentals for HCCH, HCCD, DCCD, H 13 CCH, and H 13 C 13 CH with a mean absolute error of 1.3 cm Ϫ1 , and the equilibrium rotational constant to four decimal places, without any empirical adjustment. Anharmonicity and quartic resonance constants are in excellent agreement with the recent determination of Temsamani and Herman ͓J. Chem. Phys. 103, 6371 ͑1995͔͒, except for the vibrational l-doubling constant R 45 , for which an adjustment to the computed force field is proposed.

The heats of formation of the haloacetylenes XCCY [X, Y = H, F, Cl]: basis set limitab initioresults and thermochemical analysis

“…The compression from HCCF to FCCF is still more pronounced, namely 0.0103Å at the same level of theory. (A parallel trend can be noted in the harmonic frequency for the CC stretch, which mounts from 2013 cm −1 in acetylene [40] over 2279 cm −1 in HCCF to 2527 cm…”

“…Accurate ab initio anharmonic force fields are available for acetylene [40] and HCCF [41] from the work of Martin and coworkers, and for C 2 F 2 and C 2 Cl 2 from the work of Thiel and coworkers [42,43]. From comparing the W1 and W2h entries for the ZPVE in Table II Only rather crude estimates are available for the haloacetylenes themselves (see Table II): our calculations agree with them to within the former's stated error bars, but given the magnitude thereof this is a rather hollow victory.…”

Benchmark ab initio thermochemistry of the isomers of diimide, N2H2, using accurate computed structures and anharmonic force fields

“…ethylene [66] and acetylene [67], consists of obtaining a best calculated geometry and computing ground state rotational constants from it and the anharmonic force field, then comparing the rotational constants with experiment. For good agreement with experiment in directly calculated bond distances, inclusion of inner-shell correlation is absolutely essential [48].…”

Accurate ab initio anharmonic force field and heat of formation for silane

“…As seen in At the CCSD(T)/MTcore level, we compute a bond distance of 1.4734Å, which we know from experience [66,67] should be very close to the true value. Ohno, Matsuura, Endo, and Hirota (OMEH1) [68] estimate an experimental r e bond distance of 1.4741Å…”