Isotopic and symmetry breaking effects on phosphine spectra under H → D substitutions from ab initio variational calculations

Accurate ab initio potential energy surface, rovibrational energy levels and resonance interactions of triplet ( X ~ 3 B1 ) methylene

In this work, we report rovibrational energy levels for four isotopologues of methylene (CH2 , CHD, CD2 , and 13 CH2 ) in their ground triplet electronic state (X ~ 3 B1 ) from variational calculation up to ~10,000 cm-1 and using a new accurate ab initio potential energy surface (PES). Triplet methylene exhibits a large-amplitude bending vibration and can reach a quasilinear configuration due to its low barrier (~2000 cm-1 ). To construct the ab initio PES, the Dunning's augmented correlation-consistent core-valence orbital basis sets were employed up to the sextuple-ζ quality [aug-cc-pCVXZ, X = T, Q, 5, and 6] combined with the single- and double-excitation unrestricted coupled cluster approach with a perturbative treatment of triple excitations [RHF-UCCSD(T)]. We have shown that the accuracy of the ab initio energies is further improved by including the corrections due to the scalar relativistic effects, DBOC and high-order electronic correlations. For the first time, all the available experimental rovibrational transitions were reproduced with errors less than 0.12 cm-1 , without any empirical corrections. Unlike more "traditional" nonlinear triatomic molecules, we have shown that even the energies of the ground vibrational state (000) with rather small rotational quantum numbers are strongly affected by the very pronounced rovibrational resonance interactions. Accordingly, the polyad structure of the vibrational levels of CH2 and CD2 was analyzed and discussed. The comparison between the energy levels obtained from the effective Watson A-reduced Hamiltonian, from the generating-function approach and from a variational calculation was given.

New Theoretical Infrared Line List for the Methyl Radical with Accurate Vibrational Band Origins from High-Level Ab Initio Calculations

In the present work, high-level ab initio calculations were carried out for the ground electronic state of the methyl radical (CH₃). Dunning's augmented correlation-consistent orbital basis sets were employed up to the quintuple-ζ valence quality with the core-valence electron correlation [aug-cc-pCV5Z] combined with the single- and double-excitation unrestricted coupled-cluster approach with a perturbative treatment of triple excitations [RHF-UCCSD(T)]. The explicitly correlated version of the coupled-cluster approach [RHF-UCCSD(T)-F12x{x = a, b}] was additionally applied with the core-valence cc-pCVQZ-F12 basis set in order to study convergence with respect to the basis set size. The contributions beyond the coupled-cluster level of the theory like Douglas-Kroll-Hess scalar relativistic Hamiltonian, diabatic Born-Oppenheimer corrections, and high-order electronic correlations have been included into the ab initio potential energy surfaces (PESs). It is shown that the theoretical band origins of CH₃ converge gradually to the experimental values when applying the ab initio PESs using the aug-cc-pCVXZ [X = T, Q, and 5] basis sets. For the first time, all available experimental band origins of the gaseous CH₃ are reproduced within an accuracy of 0.2 cm^-1 using a newly developed PES extrapolated to the complete basis set limit [CBS(TQ5Z)]. The reached accuracy is one order of magnitude better than that of the best available calculations. A new theoretical infrared line list was generated for astrophysical applications using an ab initio dipole moment surface computed at the RHF-UCCSD(T)/aug-cc-pCVQZ level of the theory. The manifestation of a large-amplitude motion in CH₃ is also discussed.

New Ab Initio Potential Energy Surfaces for NH3 Constructed from Explicitly Correlated Coupled-Cluster Methods

Accurate calculation of ab initio potential energy surfaces (PESs) for the NH₃ molecule is a difficult task because of the poor convergence of the standard CCSD(T) method with respect to the basis size. Currently, the best available NH₃ PESs contain empirically refined parameters. In this paper, we show that CCSD(T)/aug-cc-pCV6Z calculations are not sufficient to properly describe the PES over a large range of nuclear configurations. However, the PES obtained by the extrapolation of the CCSD(T)/aug-cc-pCVXZ (X = T, Q, 5, and 6) energies to the complete basis set limit is closer to that based on the explicitly correlated CCSD(T)-F12a method using the cc-pCVQZ-F12 orbital basis set. All of the ab initio PESs constructed in this work include the following corrections: one electron relativistic effects, diagonal Born-Oppenheimer correction, and high-order electronic correlations (CCSDT, CCSDTQ, and CCSDTQP). Finally, the root-mean-square deviation between the predicted band centers obtained from our final "pure" ab initio PES and the experimental ones in the spectral region [0-7000] cm^-1 is divided by two compared to the most accurate ab initio PES available in the literature.

A global view of isotopic effects on ro-vibrational spectra of six-atomic molecules: a case study of eleven ethylene species

In this work, we present a global view of the impact of isotopic substitutions on the spectra of eleven ethylene isotopologues obtained from variational calculations using accurate ab initio potential energy and dipole moment surfaces. This may lead to some important changes in the molecular spectra due to symmetry breaking effects lowering the initial D_2h symmetry of ¹²C₂H₄ ([triple bond, length as m-dash]¹²CH₂¹²CH₂) to C_2v, C_2h or C_s. For the very first time, we report ab initio predictions for ¹²C₂D₄ ([triple bond, length as m-dash]¹²CD₂¹²CD₂) and three C_s species: ¹²CHD¹³CH₂, ¹³CHD¹²CH₂ and ¹²C₂HD₃ ([triple bond, length as m-dash]¹²CD₂¹²CHD). To this end, we have considered the normal-mode approach based on our reduced Eckart-Watson Hamiltonian combined with ethylene ab initio surfaces. This work will contribute to the complete theoretical studies of the deuterated and ¹³C-enriched ethylene isotopologues. A total of 1252 vibrational levels are computed and all the corresponding transitions in the energy range of ≤3100 cm^-1 are predicted and compared to 151 bands assigned from experimental spectra analyses.

Theoretical rotation-vibration spectroscopy of cis- and trans-diphosphene (P2H2) and the deuterated species P2HD

Growing astronomical interest in phosphorous (P) chemistry is stimulating the search for new interstellar P-bearing molecules, a task requiring detailed knowledge of the microwave and infrared molecular spectrum. In this work, we present comprehensive rotation-vibration line lists of the cis- and trans-isomers of diphosphene (P₂H₂). The line lists have been generated using robust, first-principles methodologies based on newly computed, high-level ab initio potential energy and dipole moment surfaces. Transitions are considered between states with energies up to 8000 cm^-1 and total angular momentum J ≤ 25. These are the first-ever line lists to be reported for P₂H₂, and they should significantly facilitate future spectroscopic characterization of this system. The deuterated species trans-P₂HD and the effect of its dynamic dipole moment on the rovibrational spectrum are also discussed.