Engineering topics

Rice, Julia E.

Publications and source records attributed to Rice, Julia E..

14 records

DOE OSTI2024

Distinguishing homolytic vs heterolytic bond dissociation of phenylsulfonium cations with localized active space methods

Modeling chemical reactions with quantum chemical methods is challenging when the electronic structure varies significantly throughout the reaction and when electronic excited states are involved. Multireference methods, such as complete active space self-consistent field (CASSCF), can handle these multiconfigurational situations. However, even if the size of the needed active space is affordable, in many cases, the active space does not change consistently from reactant to product, causing discontinuities in the potential energy surface. The localized active space SCF (LASSCF) is a cheaper alternative to CASSCF for strongly correlated systems with weakly correlated fragments. The method is used for the first time to study a chemical reaction, namely the bond dissociation of a mono-, di-, and triphenylsulfonium cation. LASSCF calculations generate smooth potential energy scans more easily than the corresponding, more computationally expensive CASSCF calculations while predicting similar bond dissociation energies. Furthermore, our calculations suggest a homolytic bond cleavage for di- and triphenylsulfonium and a heterolytic pathway for monophenylsulfonium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1994

The Molecular Structure of cis-FONO

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 but in disagreement with density functional theory, cis-FONO is found to exhibit normal bond distances. The quadratic and cubic force fields of cis-FONO have also been determined in order to evaluate the effect of vibrational averaging on the molecular geometry. Vibrational averaging is found to increase bond distances, as expected, but it does not affect the qualitative nature of the bonding. The CCSD(T)/spdf harmonic frequencies of cis-FONO support our previous assertion that a band observed at 1200 /cm is a combination band (upsilon(sub 3) + upsilon(sub 4)), and not a fundamental.

Lee, Timothy J.↗

NASA NTRS1993

Ab initio study of the chlorine nitrate protonation reaction - Implications for loss of ClONO2 in the stratosphere

Ab initio quantum mechanical methods, including coupled-cluster theory, are used to determine the equilibrium geometries, dipole moments, and harmonic vibrational frequencies of ClONO2, NO2(+), and four isomers of protonated ClONO2. It was found that, for the equilibrium structures and harmonic frequencies of ClONO2, HOCl, and NO2(+), the highest-level theoretical predictions are consistent with the available experimental information concerning the reactions of ClONO2 and HOCl with HCl on the surface of polar stratospheric clouds (PSCs). The study supports a recent hypothesis that the reaction of ClONO2 on the surface of PSCs is proton catalyzed, although the mechanism is different.

Lee, Timothy J.↗

NASA NTRS1992

An extensive ab initio study of the structures, vibrational spectra, quadratic force fields, and relative energetics of three isomers of Cl2O2

Quantum mechanical computational methods are employed for an ab initio investigation of: (1) the molecular properties of the lowest isomers of the ClO dimer; and (2) predicted molecular and thermochemical properties. Techniques employed include electron correlation and particularly singles and doubles coupled-cluster (CCSD) theory with or without perturbational estimates of the effects of connected triple excitations. The isomers ClOClO and ClClO2 are found to have higher energies than the ClOOCl isomer, and the theoretical vibrational frequencies of the isomers are well correlated with experimental data. Experimental values of the heat of formation for the isomers are also compared with calculations based on an isodesmic reaction with Cl2O, H2O, and HOOH.

Lee, Timothy J.↗

NASA NTRS1992

FONO - A difficult case for theory and experiment

High levels of ab initio theory are used to investigate the equilibrium structures, vibrational spectra, and relative energetics of FNO2, cis-FONO, and trans-FONO isomers. FNO2 is determined to be 36.9 +/- 2.5 kcal/mol (0 K) more stable than cis-FONO, which is more stable than trans-FONO by 2.5 +/- 1.0 kcal/mol (0 K). The molecular structure of cis-FONO is shown to have typical F-O and central O-N single bond distances, indicating that cis-FONO has a structure analogous to HONO. The computed vibrational spectrum of cis-FONO is shown to be consistent with IR matrix isolation experiments, including isotopic shifts. The experimentally deduced structure of FONO is shown to be incorrect because one of the vibrational bands included in the normal coordinate analysis is either a combination band or an overtone.

Lee, Timothy J.↗

NASA NTRS1992

Connected triple excitations in coupled-cluster calculations of hyperpolarizabilities: Neon

We have calculated the second hyperpolarizability gamma of neon using the CCSD(T) method. The accuracy of the CCSD(T) approach has been established by explicit comparison with the single, double and triple excitation coupled-cluster (CCSDT) method using extended basis sets that are known to be adequate for the description of gamma. Our best estimate for gamma(sub 0) of 110 +/- 3 a.u. is in good agreement with other recent theoretical values and with Shelton's recent experimental estimate of 108 +/- 2 a.u. Comparison of the MP2 and CCSD(T) hyperpolarizability values indicates that MP2 gives a very good description of the electron correlation contribution to gamma(sub 0). We have combined MP2 frequency-dependent corrections with the CCSD(T) gamma(sub 0) to yield values of gamma(-2 omega;omega,omega,0) and gamma(exp K)(-omega;omega,0,0).

Rice, Julia E.↗

NASA NTRS1992

Proton affinity of methyl nitrate - Less than proton affinity of nitric acid

Several state-of-the-art ab initio quantum mechanical methods were used to investigate the equilibrium structure, dipole moments, harmonic vibrational frequencies, and IR intensities of methyl nitrate, methanol, and several structures of protonated methyl nitrate, using the same theoretical methods as in an earlier study (Lee and Rice, 1992) of nitric acid. The ab initio results for methyl nitrate and methanol were found to be in good agreement with available experimental data. The proton affinity (PA) of methyl nitrate was calculated to be 176.9 +/-5 kcal/mol, in excellent agreement with the experimental value 176 kcal/mol obtained by Attina et al. (1987) and less than the PA value of nitric acid. An explanation of the discrepancy of the present results with those of an earlier study on protonated nitric acid is proposed.

Lee, Timothy J.↗

NASA NTRS1992

Ab initio study of the molecular structure and vibrational spectrum of nitric acid and its protonated forms

The equilibrium structures, harmonic vibrational frequencies, IR intensities, and relative energetics of HNO3 and its protonated form H2NO3+ were investigated using double-zeta plus polarization and triple-zeta plus polarization basis sets in conjunction with high-level ab initio methods. The latter include second-order Moller-Plesset perturbation theory, the single and double excitation coupled cluster (CCSD) methods, a perturbational estimate of the effects of connected triple excitations (CCSD(T)), and the self-consistent field. To determine accurate energy differences CCSD(T) energies were computed using large atomic natural orbital basis sets. Four different isomers of H2NO3+ were considered. The lowest energy form of protonated nitric acid was found to correspond to a complex between H2O and NO2+, which is consistent with earlier theoretical and experimental studies.

Lee, Timothy J.↗

NASA NTRS1991

A theoretical study of Na(H2O)n(+) (n = 1-4)

The successive H2O binding energies of Na(H2O)n(+) are in excellent agreement with experiment and are rather insensitive to electron correlation since the bonding is predominantly electrostatic. A point-charge model shows that changes in the successive binding energies are due primarily to ligand-ligand repulsion. Vibrational frequencies and IR intensities are determined for Na(H2O)n(+) (n = 1-4) at the self-consistent-field and second-order Moller-Plesset levels of theory to facilitate experimental study of these ions.

Bauschlicher, Charles W., Jr.↗

NASA NTRS1991

The energy separation between the classical and nonclassical isomers of protonated acetylene - An extensive study in one- and n-particle space saturation

The energy separation between the classical and nonclassical forms of protonated acetylene has been reinvestigated in light of the recent experimentally deduced lower bound to this value of 6.0 kcal/mol. The objective of the present study is to use state-of-the-art ab initio quantum mechanical methods to establish this energy difference to within chemical accuracy (i.e., about 1 kcal/mol). The one-particle basis sets include up to g-type functions and the electron correlation methods include single and double excitation coupled-cluster (CCSD), the CCSD(T) extension, multireference configuration interaction, and the averaged coupled-pair functional methods. A correction for zero-point vibrational energies has also been included, yielding a best estimate for the energy difference between the classical and nonclassical forms of 3.7 + or - 1.3 kcal/mol.

Lindh, Roland↗

NASA NTRS1991

The determination of accurate dipole polarizabilities alpha and gamma for the noble gases

Accurate static dipole polarizabilities alpha and gamma of the noble gases He through Xe were determined using wave functions of similar quality for each system. Good agreement with experimental data for the static polarizability gamma was obtained for Ne and Xe, but not for Ar and Kr. Calculations suggest that the experimental values for these latter ions are too low.

Rice, Julia E.↗

NASA NTRS1991

Theoretical characterization of tetrahedral N4

Results are reported from ab initio theoretical calculations of the structure and energetics of tetrahedral N4. The theoretical basis and implementation of the computations are reviewed, and the results are presented in tables. It is found that a tetrahedral N4 molecule has energy 186 kcal/mole greater than that of two N2 molecules, with a 61-kcal/mole barrier between the two species. Brief consideration is given to the possible use of N4 (should a process for synthesizing it be developed) as a high-energy-density fuel for supersonic transport aircraft.

Lee, Timothy J.↗

NASA NTRS1989

The polarizabilities of neon

The static dipole polarizability, second hyperpolarizability, quadrupole polarizability and dipole-dipole-quadrupole hyperpolarizability for the Ne atom are computed, using large Gaussian basis sets and coupled cluster wavefunctions. Correlation effects are substantial, ranging from 15 percent for the dipole polarizability to 40 percent for the second hyperpolarizability. It is confirmed by explicit calculation earlier hypotheses that g-type and higher angular momentum functions and core correlation effects make almost no contribution to these properties in Ne. Triple excitations, on the other hand, are very important, accounting for as much as 25 percent of the correlation contribution to the second hyperpolarizability. The best estimate of the second hyperpolarizability is 119 + or - 4 au, in good agreement with the recent calculations of Maroulis and Thakkar (1989) and the latest experimental result of Shelton (1989).

Taylor, Peter R.↗

NASA NTRS1989

The determination of accurate dipole polarizabilities alpha and gamma for the noble gases

The static dipole polarizabilities alpha and gamma for the noble gases helium through xenon were determined using large flexible one-particle basis sets in conjunction with high-level treatments of electron correlation. The electron correlation methods include single and double excitation coupled-cluster theory (CCSD), an extension of CCSD that includes a perturbational estimate of connected triple excitations, CCSD(T), and second order perturbation theory (MP2). The computed alpha and gamma values are estimated to be accurate to within a few percent. Agreement with experimental data for the static hyperpolarizability gamma is good for neon and xenon, but for argon and krypton the differences are larger than the combined theoretical and experimental uncertainties. Based on our calculations, we suggest that the experimental value of gamma for argon is too low; adjusting this value would bring the experimental value of gamma for krypton into better agreement with our computed result. The MP2 values for the polarizabilities of neon, argon, krypton and zenon are in reasonabe agreement with the CCSD and CCSD(T) values, suggesting that this less expensive method may be useful in studies of polarizabilities for larger systems.

Rice, Julia E.↗