Christopher E. Dateo

An Accurate Quartic Force Field and Vibrational Frequencies for HNO and DNO

An accurate ab initio quartic force field for HNO has been determined using the singles and doubles coupled‐cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), in conjunction with the correlation consistent polarized valence triple zeta (cc‐pVTZ) basis set. Improved harmonic frequencies were determined with the cc‐pVQZ basis set. Fundamental vibrational frequencies were determined using a second‐order perturbation theory analysis and also using variational calculations. The N–O stretch and bending fundamentals are determined well from both vibrational analyses. The H–N stretch, however, is shown to have an unusually large anharmonic correction, and is not well determined using second‐order perturbation theory. The H–N fundamental is well determined from the variational calculations, demonstrating the quality of the ab initio quartic force field. The zero‐point energy of HNO that should be used in isodesmic reactions is also discussed.

Quantal study of the exchange reaction for N+N2 using an ab initio potential energy surface

The N+N2 exchange rate is calculated using a time-dependent quantum dynamics method on a newly determined ab initio potential energy surface (PES) for the ground 4A″ state. This ab initio PES shows a double barrier feature in the interaction region with the barrier height at 47.2 kcal/mol, and a shallow well between these two barriers, with the minimum at 43.7 kcal/mol. A quantum dynamics wave packet calculation has been carried out using the fitted PES to compute the cumulative reaction probability for the exchange reaction of N+N2(J=0). The J–K shift method is then employed to obtain the rate constant for this reaction. The calculated rate constant is compared with experimental data and a recent quasiclassical calculation using a London–Eyring–Polanyi–Sato PES. Significant differences are found between the present and quasiclassical results. The present rate calculation is the first accurate three-dimensional quantal dynamics study for the N+N2 reaction system and the ab initio PES reported here is the ...

A global ab initio potential for HCN/HNC, exact vibrational energies, and comparison for experiment

Abstract An ab initio, i.e. from first principles, calculation of vibrational energies of HCN and HNC is reported. The vibrational calculations were done with a new potential derived from a fit to 1124 ab initio electronic energies, which were calculated using the highly accurate CCSD(T) coupled-cluster method in conjunction with a large atomic natural orbital basis set. The properties of this potential are presented, and the vibrational calculations are compared to experiment for 54 vibrational transitions, 39 of which are for zero total angular momentum, J = 0, and 15 of which are for J = 1. The level of agreement with experiment is unprecedented for a triatomic with two non-hydrogen atoms, and demonstrates the capability of the latest computational methods to give reliable predictions on a strongly bound triatomic molecule at very high levels of vibrational excitation.

An analysis of chlorine and bromine oxygen bonding and its implications for stratospheric chemistry

Trends in chlorine/oxygen and bromine/oxygen single and double bonds are examined for several molecules of interest in stratospheric halogen chemistry. Specifically, the relationships between bond distance and quadratic force constant, and bond distance and ionic bonding character are examined, together with bond energies. Similar to a previous study of FO bonding, it is found that the relationship between bond distance and force constant for Cl—O and Br—O single bonds is unusual and distinctly nonlinear. This is attributed to the through space interaction of halogen lone-pair electrons with the remainder of the molecule. Supporting evidence for this assertion is given by the fact that for X=O (X is Cl, Br) double bonds, where there are fewer halogen lone-pair electrons due to hypervalent bonding, this relationship is approximately linear. A detailed explanation for chlorine and bromine hypervalent bonding is presented which is consistent with all available data and with the trends studied here. In this m...

The molecular structure of cis-FONO

Abstract The molecular structure of cis-FONO has been determined with the CCSD(T) correlation method using an spdf quality basis set. In agreement with previous coupled-cluster calculations cis-FONO exhibits normal bond distances. The cubic force field of cis-FONO has been determined in order to evaluate the effect of vibrational averaging on the geometry. Vibrational averaging is found to increase bond distances, as expected, but it does not affect the qualitative description of the bonding. The CCSD(T)/spdf harmonic frequencies of cis-FONO support our previous assertion that a band observed at 1200 cm −1 is a combination band (ν 3 + ν 4 ), and not a fundamental. In contrast to earlier density functional theory (DFT) studies, calculations using an empirically optimized hybrid functional give a structure and vibrational frequencies for cis-FONO that are in qualitative agreement with the CCSD(T) values, resolving the discrepancy between DFT and CCSD(T).

Ab Initio Vibrational Levels For HO2 and Vibrational Splittings for Hydrogen Atom Transfer

We calculate vibrational levels and wave functions for HO2 using the recently reported ab initio potential energy points of Walch and Duchovic [J. Chem. Phys. 94, 7068 (1991)] as fit by Dateo (unpublished). There is intramolecular hydrogen atom transfer when the hydrogen atom tunnels through a T‐shaped saddle point separating the two equivalent equilibrium geometries, and correspondingly, the vibrational levels are split. We focus on vibrational levels and wave functions with significant splitting. The first three vibrational levels with splitting greater than 2 cm−1 are (1,5,0), (0,7,1), and (0,8,0), where v2 is the O–O–H bend quantum number. We discuss the dynamics of hydrogen atom transfer; in particular, the O–O distances at which hydrogen atom transfer is most probable for these vibrational levels.

The effect of approximating some molecular integrals in coupled-cluster calculations: fundamental frequencies and rovibrational spectroscopic constants for isotopologues of cyclopropenylidene

The effect of approximating the three- and four-virtual molecular orbital integrals in single and double coupled-cluster theory including a perturbational correction for connected triple excitations [CCSD(T)] is investigated for the calculation of higher-order properties, specifically the calculation of a molecular quartic force field and spectroscopic constants. The approach was proposed previously, but investigated for only second- and lower-order properties. It is shown that the conclusions reached previously are essentially unchanged on moving to higher-order properties. That is, approximating the selected integrals has essentially no effect on the accuracy of CCSD(T) calculations, and the error due to approximating integrals is much smaller than the residual error due to one-particle basis set deficiencies. The advantage of this approach is that it significantly reduces the amount of data needed to perform CCSD(T) calculations, thereby reducing computational requirements associated with input/output operations and message passing in massively parallel, distributed memory algorithms. These savings are particularly important for large basis set calculations where the reduction in data can be as high as three orders of magnitude for ∼1000 unoccupied molecular orbitals. The approach was tested by computing the quartic force field, vibrational frequencies, and spectroscopic constants of cyclopropenylidene and isotopologues. Comparison of our best results with available experimental data shows excellent agreement between theory and experiment. It is hoped that the theoretical spectroscopic data presented herein for cyclopropenylidene and isotopologues is useful in the interpretation of future laboratory experiments and astronomical observations.

Towards the synthesis of the high energy density material TdN4: excited electronic states

Abstract Vertical electronic excitation energies for singlet states have been computed for the high energy density material T d N 4 in order to assess synthetic routes that originate from excited states of N 2 molecules. Based on linear response coupled-cluster calculations, the lowest six excited states are 9.35(1 1 T 1 ), 10.01(1 1 T 2 ), 10.04(1 1 A 2 ), 10.07(1 1 E ), 10.12(2 1 T 1 ), and 10.42(2 1 T 2 ) eV above the ground state. Comparison with the energies of excited states of N 2 +N 2 fragments, leads us to propose that the most likely synthetic route for T d N 4 involving this mechanism arises from combination of two bound quintet states of N 2 .

AN ACCURATE AB INITIO QUARTIC FORCE FIELD AND VIBRATIONAL FREQUENCIES FOR CYCLOPROPENYLIDENE

Abstract The singles and doubles coupled-cluster method that includes a perturbative correction for connected triple excitations, denoted CCSD(T), is used in conjunction with an spdf quality one-particle basis set to determine an accurate quartic force field for cyclopropenylidene. A second-order perturbation theory treatment of vibrational anharmonicities, together with proper treatment of Fermi resonances, is used to predict fundamental vibrational frequencies of cyclopropenylidene and its 13C and deuterium isotopomers. Agreement between theory and the available experimental data is excellent. It is demonstrated that four vibrational bands assigned to cyclopropenylidene in 1984 matrix isolation experiments are correct, contrary to a recent suggestion. The anharmonic progression in the C-H stretches is examined and found to be similar for both the symmetric and antisymmetric C-H stretches, contrary to the findings from another recent study.

Accurate spectroscopic characterization of 12C14N−, 13C14N−, and 12C15N−

It is argued that should molecular anions be ubiquitous in the interstellar medium, that current models of the chemical processing of the biogenic elements would have to be fundamentally changed. The lack of high-resolution spectroscopic constants for molecular anions from laboratory experiments is one possible reason that no molecular anions have ever been positively identified from vibrational or rotational spectroscopic astronomical observations. The current theoretical study provides highly accurate predictions for the spectroscopic constants of CN− in the hopes that they will be useful in the identification of CN− from astronomical observations. The singles and doubles coupled-cluster method that includes a perturbational correction for connected triple excitations, denoted CCSD(T), is used in conjunction with several one-particle basis sets to determine quartic force fields for CN−. Basis set convergence properties of the various rovibrational spectroscopic constants are studied. To assure numerical stability of the quartic force fields and to investigate variational calculations of vibrational energy levels, a sextic force field has also been evaluated. The largest one-particle basis set employed contains up through h functions on carbon and nitrogen. Analogous calculations have been carried out on neutral CN and comparison to experiment allows highly accurate predictions for the spectroscopic constants of 12Cl4N−, 13Cl4N−, and 12Cl5N− to be made. For 12Cl4N−, B0 is predicted to be 1.87158±0.00090 cm−1 and ν is predicted to occur at 2042±3 cm−l.