High-Level Coupled-Cluster Study on Substituent Effects in H2 Activation by Low-Valent Aluminyl Anions.

The synthesis of novel aluminyl anion complexes has been well exploited in recent years. Moreover, the elucidation of the structure and reactivity of these complexes opens the path toward a new understanding of low-valent aluminum complexes and their chemistry. This work computationally treats the substituent effect on aluminyl anions to discover suitable alternatives for H2 activation at a high level of theory utilizing coupled-cluster techniques extrapolated to the complete basis set. The results reveal that the simplest AlH2- system is the most reactive toward the activation of H2, but due to the low steric demand, severe difficulty in the stabilization of this system makes its use nonviable. However, the results indicate that, in principle, aluminyl systems with -C, -CN, -NC, and -N chelating centers would be the best choices of ligand toward the activation of molecular hydrogen by taking care of suitable steric demand to prevent dimerization of the catalysts. Furthermore, computations show that monosubstitution (besides -H) in aluminyl anions is preferred over disubstitution. So our predictions show that bidentate ligands may yield less reactive aluminyl anions to activate H2 than monodentate ones.

Substituent Effects on Aluminyl Anions and Derived Systems: A High-Level Theory.

Aluminyl anions are low-valent aluminum species bearing a lone pair of electrons and a negative charge. These systems have drawn recent synthetic interest for their nucleophilic nature, allowing for the activation of σ-bonds, and have been proposed as a pathway to hydrogen energy storage. In this research, we provide high-level ab initio geometries and energies for both the simplest aluminyl anion (AlH2-) and several substituted derivatives. Geometries are reported using the gold-standard CCSD(T)/aug-cc-pV(T+d)Z level of theory. Energies were extrapolated to the complete basis set limit through the focal point approach, utilizing coupled-cluster methods through perturbative quadruples and basis sets up to five-ζ quality. Geometries were rationalized using electrostatic, steric, and orbital donation effects. The donation from substituents to Al is accompanied by back-donation effects, a property traditionally thought of in transition-metal systems. Stereoelectronic effects through the secondary orbital interaction play a fundamental role in stabilizing these low-valent aluminum compounds and would likely also affect the feasibility of their use within several industrial applications. The energetic analysis of the formation of each substituted anion is rationalized as the result of three energetic schemes. The effectiveness of these schemes for determining the relative formation energies is discussed.

Relatives of cyanomethylene: replacement of the divalent carbon by B-, N+, Al-, Si, P+, Ga-, Ge, and As.

The lowest lying singlet and triplet states of HBCN-, HCCN, HNCN+, HAlCN-, HSiCN, HPCN+, HGaCN-, HGeCN, and HAsCN+ were studied using the CCSDT(Q)/CBS//CCSD(T)/aug-cc-pVQZ level of theory. Periodic trends in geometries, singlet-triplet gaps, and barriers to linearity were established and analyzed. The first row increasingly favors the triplet state, with a singlet-triplet gap (ΔEST = Esinglet - Etriplet) of 3.5 kcal mol-1, 11.9 kcal mol-1, and 22.6 kcal mol-1, respectively, for HBCN-, HCCN, and HNCN+. The second row increasing favors the singlet state, with singlet-triplet gaps of -20.4 kcal mol-1 (HAlCN-), -26.6 kcal mol-1 (HSiCN), and -26.8 kcal mol-1 (HPCN+). The third row also favors the singlet state, with singlet-triplet gaps of -26.8 kcal mol-1 (HGaCN-), -33.5 kcal mol-1 (HGeCN), and -33.1 kcal mol-1 (HAsCN+). The HXCN species have larger absolute singlet-triplet energy gaps compared to their parent species XH2 except for the case of X = N+. The effect of the substitution of hydrogen with a cyano group was analyzed with isodesmic bond separation analysis and NBO.

Important features of the potential energy surface of the methylamine plus O(1D) reaction.

This research presents an ab initio characterization of the potential energy surface for the methylamine plus 1D oxygen atom reaction, which may be relevant to interstellar chemistry. Geometries and harmonic vibrational frequencies were determined for all stationary points at the CCSD(T)/aug-cc-pVTZ level of theory. The focal point method along with several additive corrections was used to obtain reliable CCSDT(Q)/CBS potential energy surface features. Extensive conformational analysis and intrinsic reaction coordinate computations were performed to ensure accurate chemical connectivity of the stationary points. Five minima were determined to be possible products of this reaction and three novel transition states were found that were previously unreported or mislabeled in the literature. The pathways we present can be used to guide further searches for NH2 containing species in the interstellar medium.

Fundamental Vibrational Analyses of the HCN Monomer, Dimer and Associated Isotopologues.

In this work we provide high level ab initio treatments of the structures, vibrational frequencies, and electronic energies of the HCN monomer and dimer systems along with several isotopologues. The plethora of information related to this system within the literature is summarized and serves as a basis for comparison with the results of this paper. The geometry of the dimer and monomer are reported at the all electroncoupled-cluster singles, doubles, and perturbative triples level of theory [AE-CCSD(T)] with the correlation consistent quadruple-zeta quality basis sets with extra core functions (cc-pCVQZ) from Dunning. The theoretical geometries and electronic structures are further analyzed through the use of the Natural Bond Orbital (NBO) method and Natural Resonance Theory (NRT). At the AE-CCSD(T)/cc-pCVQZ level of theory, the full cubic with semi-diagonal quartic force field for nine dimer and four monomer isotopologues (the parent isotopologue along with 15 N, 13 C, and D derivatives) were obtained to treat the anharmonicity of the vibrations via second order vibrational perturbation theory (VPT2). Lastly, the enthalpy change associated with the formation of the dimer from two monomer units was determined using the focal point analysis. Computations including coupled-cluster through perturbative quadruples as well as basis sets up to six-zeta quality, including core functions (cc-pCVXZ, X=D,T,Q,5,6) were used to extrapolate to the AE-CCSDT(Q)/CBS energy associated with this hydrogen-bond forming process. After appending anharmonic zero-point vibrational, relativistic, and diagonal Born-Oppenheimer corrections, we report a value of -3.93 kcal mol-1 for the enthalpy of formation. To our knowledge, each set of results (geometries, vibrational frequencies, and energetics) reported in this study represents the highest-level and most reliable theoretical predictions reported for this system.

The bismuth tetramer Bi4: the ν3 key to experimental observation.

The spectroscopic identification of Bi4 has been very elusive. Two constitutional Bi4 isomers of Td and C2v 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 Td 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 Bi4 (Td) into two Bi2 and the adiabatic energy difference between the C2v and Td isomers of Bi4. These quantities are predicted to be +65 kcal mol-1 and +39 kcal mol-1, respectively. Two electron vertical excitation energies between the Td and C2v electronic configurations are computed to be 156 kcal mol-1 for the Td isomer and 9 kcal mol-1 for the C2v isomer. The most probable approach to laboratory spectroscopic identification of Bi4 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 Td isomer. The moderate IR intensity for the only allowed fundamental may explain why Bi4 has yet to be observed. Through natural bond orbital analysis, the C2v isomer of Bi4 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 C2v isomer.

Investigating the ground-state rotamers of n-propylperoxy radical.

The n-propylperoxy radical has been described as a molecule of critical importance to studies of low temperature combustion. Ab initio methods were used to study this three-carbon alkylperoxy radical, normal propylperoxy. Reliable CCSD(T) (coupled-cluster theory, incorporating single, double, and perturbative triple)/ANO0 geometries were predicted for the molecule's five rotamers. For each rotamer, energetic predictions were made using basis sets as large as the cc-pV5Z in conjunction with coupled cluster levels of theory up to CCSDT(Q). Along with the extrapolations, corrections for relativistic effects, zero-point vibrational energies, and diagonal Born-Oppenheimer corrections were used to further refine energies. The results indicate that the lowest conformer is the gauche-gauche (GG) rotamer followed by the gauche-trans (0.12 kcal mol-1 above GG), trans-gauche (0.44 kcal mol-1), gauche'-gauche (0.47 kcal mol-1), and trans-trans (0.57 kcal mol-1). Fundamental vibrational frequencies were obtained using second-order vibrational perturbation theory. This is the first time anharmonic frequencies have been computed for this system. The most intense IR features include all but one of the C-H stretches. The O-O fundamental (1063 cm-1 for the GG structure) also has a significant IR intensity, 19.6 km mol-1. The anharmonicity effects on the potential energy surface were also used to compute vibrationally averaged rg,0K bond lengths, accounting for zero-point vibrations present within the molecule.