The global minima of urea and thiourea were characterized along with other low‐lying stationary points. Each structure was optimized with the CCSD(T) method and triple‐
The
- NSF-PAR ID:
- 10461879
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- International Journal of Quantum Chemistry
- Volume:
- 119
- Issue:
- 8
- ISSN:
- 0020-7608
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract ζ correlation consistent basis sets followed by harmonic vibrational frequency computations. Relative energies evaluated near the complete basis set limit with both canonical and explicitly correlated CCSD(T) techniques reveal several subtle but important details about both systems. These computations resolve a discrepancy by demonstrating that the electronic energy of the C2v second‐order saddle point of urea lies at least 1.5 kcal mol−1above the C2global minimum regardless of whether the structures were optimized with MP2, CCSD, or CCSD(T). Additionally, urea effectively has one minimum instead of two because the electronic barrier for inversion at one amino group in the Cslocal minimum vanishes at the CCSD(T) CBS limit. Characterization of both systems with the same ab initio methods and large basis sets conclusively establishes that the electronic barriers to inversion at one or both NH2groups in thiourea are appreciably smaller than in urea. CCSDT(Q)/cc‐pVTZ computations show higher‐order electron correlation effects have little impact on the relative energies and are consistently offset by core correlation effects of opposite sign and comparable magnitude. -
Insight into the Binding of Argon to Cyclic Water Clusters from Symmetry-Adapted Perturbation Theorymore » « less
This work systematically examines the interactions between a single argon atom and the edges and faces of cyclic H2O clusters containing three–five water molecules (Ar(H2O)n=3–5). Full geometry optimizations and subsequent harmonic vibrational frequency computations were performed using MP2 with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). Optimized structures and harmonic vibrational frequencies were also obtained with the two-body–many-body (2b:Mb) and three-body–many-body (3b:Mb) techniques; here, high-level CCSD(T) computations capture up through the two-body or three-body contributions from the many-body expansion, respectively, while less demanding MP2 computations recover all higher-order contributions. Five unique stationary points have been identified in which Ar binds to the cyclic water trimer, along with four for (H2O)4 and three for (H2O)5. To the best of our knowledge, eleven of these twelve structures have been characterized here for the first time. Ar consistently binds more strongly to the faces than the edges of the cyclic (H2O)n clusters, by as much as a factor of two. The 3b:Mb electronic energies computed with the haTZ basis set indicate that Ar binds to the faces of the water clusters by at least 3 kJ mol−1 and by nearly 6 kJ mol−1 for one Ar(H2O)5 complex. An analysis of the interaction energies for the different binding motifs based on symmetry-adapted perturbation theory (SAPT) indicates that dispersion interactions are primarily responsible for the observed trends. The binding of a single Ar atom to a face of these cyclic water clusters can induce perturbations to the harmonic vibrational frequencies on the order of 5 cm−1 for some hydrogen-bonded OH stretching frequencies.
-
more » « less
The optimized geometries, vibrational frequencies, and dissociation energies from MP2 and CCSD(T) computations with large correlation consistent basis sets are reported for (H2S)2and H2O/H2S. Anharmonic vibrational frequencies have also been computed with second‐order vibrational perturbation theory (VPT2). As such, the fundamental frequencies, overtones, and combination bands reported in this study should also provide a useful road map for future spectroscopic studies of the simple but important heterogeneous H2O/H2S dimer in which the hydrogen bond donor and acceptor can interchange, leading to two unique minima with very similar energies. Near the CCSD(T) complete basis set limit, the HOH⋯SH2configuration (H2O donor) lies only 0.2 kcal mol−1below the HSH⋯OH2structure (H2S donor). When the zero‐point vibrational energy is included, however, the latter configuration becomes slightly lower in energy than the former by <0.1 kcal mol−1. © 2018 Wiley Periodicals, Inc.
-
The spectroscopic identification of Bi 4 has been very elusive. Two constitutional Bi 4 isomers of T d and C 2v symmetry are investigated and each is found to be a local energetic minimum. The optimized geometries and vibrational frequencies of these two isomers are obtained at the CCSD(T)/cc-pVQZ-PP level of theory, utilizing the Stoll, Metz, and Dolg 60-electron effective core potential. The fundamental frequencies of the T d isomer are obtained at the same level of theory. The focal point analysis method, from a maximum basis set of cc-pV5Z-PP, and proceeding to a maximum correlation method of CCSDTQ, was employed to determine the dissociation energy of Bi 4 ( T d ) into two Bi 2 and the adiabatic energy difference between the C 2v and T d isomers of Bi 4 . These quantities are predicted to be +65 kcal mol −1 and +39 kcal mol −1 , respectively. Two electron vertical excitation energies between the T d and C 2v electronic configurations are computed to be 156 kcal mol −1 for the T d isomer and 9 kcal mol −1 for the C 2v isomer. The most probable approach to laboratory spectroscopic identification of Bi 4 is via an infrared spectrum. The predicted fundamentals (cm −1 ) with harmonic IR intensities in parentheses (km mol −1 ) are 94(0), 123(0.23), and 167(0) for the T d isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi 4 has yet to be observed. Through natural bond orbital analysis, the C 2v isomer of Bi 4 was discovered to exhibit “long-bonding” between the furthest apart ‘wing’ atoms. This long-bonding is postulated to be facilitated by the σ-bonding orbital between the ‘spine’ atoms of the C 2v isomer.more » « less
-
computational models are validated against various experimental data in order to assess overall accuracy and identify the method most apt for a given situation. In my research group, we have often validated quantum-chemical structure predictions against experimental gas-phase geometries. However, these structures are formally different; the experimental results reflect average distances that result upon ground-state vibrational averaging, whereas the theory predicts equilibrium geometries. Almost always, this distinction is trivial, and beyond the precision afforded by either experiment or theory. In one rather noteworthy instance, however, the CH3CN–BF3 complex, we demonstrated that there was a genuine, and rather significant difference between the experimental and theoretical geometries; the experimental B-N distance that was nearly 0.2 Å longer than that in the theoretically-determined equilibrium geometry. This resulted from an extreme anharmonicity in the donor-acceptor (B-N) potential, which manifested a significant asymmetry in ground state vibrational wavefunction. In our recent work, we have a similar, albeit more subtle trend in several other donor-acceptor systems including H2O–SO3, H3N–SiF4, C6H5N–SO2, and H3N–SO3. What we have noted specifically, is that the predicted donor-accptor bond lengths among several DFT and post-HF methods (with the aug-cc-pVTZ and aug-cc-pVQZ basis sets) are incredibly consistent, yet they differ from the experimental bond lengths to an extent that exceeds the quoted uncertainty. For pyridine-SO2, these differences rival CH3CN–BF3 (about 0.2) and in fact, the shape of the donor-acceptor potentials are quite similar. The other cases are more subtle, with experiment-theory differences of several hundredths of an angstrom. Yet the donor-acceptor potential curves in these systems are significantly anharmonic. The extent of the asymmetry in the ground vibrational wavefunction for the donor-acceptor stretching mode will be explored explicitly, in order to quantitatively assess the differences between the equilibrium and vibrationally averaged structures.more » « less
