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Electronic transitions are observed for the O 2 + –Ar and O 2 + –N2 complexes over the 225–350 nm range. The transitions are not associated with recognized electronic band systems of the respective atomic and diatomic constituents (Ar+, Ar, O 2 + , O2, N 2 + , and N2) but rather are due to charge transfer transitions. Onsets of the O 2 + –Ar and O 2 + –N2 band systems occur at 3.68 and 3.62 eV, respectively, corresponding to the difference in the ionization potentials of Ar and O2 (3.69 eV), and N2 and of O2 (3.51 eV), suggesting the band systems arise from intramolecular charge transfer transitions to states correlating with O2 (X 3 Σ g − ) + Ar+ (2Pu) and O2 (X 3 Σ g − ) + N 2 + (X 2 Σ g + ) limits, respectively. The dominant vibronic progressions have ωe values of 1565 cm−1 for O 2 + –Ar and 1532 cm−1 for O 2 + –N2, reasonably close to the value for the neutral O2 molecule in its X 3 Σ g − state (1580 cm−1). Higher energy band systems for O 2 + –Ar and O 2 + –N2 are assigned to transitions to states correlating with the O2 (a1Δg) + Ar+ (2Pu) and O2 (a1Δg) + N 2 + (X 2 Σ g + ) limits, respectively. [ABSTRACT FROM AUTHOR]

To date, four $ \hbox {C}_2\hbox {H}_4\hbox {O}_2 $ C 2 H 4 O 2 isomers have been identified in the interstellar medium: acetic acid, glycolaldehyde, methyl formate, and (Z)-1,2-ethenediol. The detection of these species raises the question of whether other related carbohydrates could be present in similar astrophysical environments. In this study, we report a systematic investigation of six $ \hbox {C}_2\hbox {H}_4\hbox {O}_2 $ C 2 H 4 O 2 isomers not previously observed spectroscopically in space: anti-(1)-, (2)- and (3)- $ \hbox {CH}_2=\hbox {CHOOH} $ CH 2 = CHOOH , methyldioxirane, and anti- and syn-(2S)-oxiran-2-ol. For each species, we employ coupled-cluster theory at the AE-CCSD(T)/cc-pCVQZ level to provide accurate equilibrium parameters, rotational constants, quartic centrifugal distortion parameters, and permanent electric dipole moment components. Anharmonic force fields were determined within vibrational perturbation theory to second order and evaluated using the MP2 method, thus allowing the evaluation of vibrational corrections. These computations provided, as a by-product, sextic centrifugal distortion constants. Rotational spectra have subsequently been simulated in the millimeter/submillimeter-wave frequency for the first time. It is hoped and anticipated that the accurate theoretical rovibrational spectral data presented herein will aid in experimental investigations of these title molecules in the interstellar medium. [ABSTRACT FROM AUTHOR]

We have investigated the structure and spectra (NMR, UV/visible) of several Yariv reagents and models for these species, using both density functional theory (DFT) and approximate coupled‐cluster methods (ADC(2)). We have also studied the formation of dimers and model trimers as a first step to understand the aggregation of these species in aqueous solution. One question that our calculations help resolve is the origin of two strong bands in the visible region of the spectrum: these are confirmed as resulting from enol‐keto tautomerism, and are not a result of aggregation, which was a hypothesis that had been advanced previously. The UV/visible spectrum of the Yariv reagents can be explained entirely in terms of tautomerism within a single molecule – no dimers or clusters are involved. [ABSTRACT FROM AUTHOR]

The molecular species C3H4O represents a striking example of an astrochemical conundrum. With more than 60 structural isomers theoretically possible, to date only acrolein (CH2CHCHO) has been identified in the Sgr B2(N) region of the interstellar medium (ISM). The topography of the singlet potential energy surface is complicated, with three low-lying minima predicted to be almost isoenergetic: cis and trans-acrolein, and methylketene (CH3CHCO). Our CCSD(T)/cc-pVTZ calculations confirm that methylketene is energetically lower than cis-acrolein, lying only 1.9 kJ mol−1 above the trans-isomer, which is the global minimum. In this respect, methylketene is a promising candidate for interstellar observation. Unfortunately, however, despite several searches its astronomical detection has been unsuccessful. To this end, the key question is whether in fact methylketene exists as a discrete chemical entity in the ISM at all? In this paper, we present a detailed examination of the C3H4O potential energy surface, with specific focus on formation pathways. CCSD(T)/cc-pVTZ calculations enable a more elaborate interpretation of reaction mechanisms than was published hitherto. Our results show that gauche-propargyl alcohol and syn and anti-allenol emerge as interesting new targets for observational astronomers in TMC-1: given the recent discovery of the propargyl radical in this region, barrierless product channels involving OH˙ lend support to their candidacy as possible interstellar species. Finally, this work provides accurate spectral data of these three potential molecules, to be used for searches in interstellar space. [ABSTRACT FROM AUTHOR]

Functional group contributions from ab initio calculations were the basis for zero-point vibrational effects on proton shieldings.

A theoretical study of the optical characteristics of the chromone molecule using multireference perturbation theory and density functional theory, is presented.

It has been possible to compute the scalar relativistic contribution to the atomization energies of BF3. Scalar relativistic effects cut the atomization energy of BF3 by around 0.7 kcal/mol.

Research was conducted to address questions on the heat of vaporization of silicon through a benchmark calculation of the total atomization energy of SiF4 molecule. Results supported the suggestion of Grev and Schaefer that the uncertain JANAF/CODATA value of 106.5+1-1.9 kcal/mol must be revised to a lower value. Future studies on heats produced by silicon compound formation could take advantage of the revision.

Methylketene (CH 3 CHCO) and acrolein (CH 2 CHCHO) have received considerable attention from both theoreticians and experimentalists. In a number of computational studies, it has been identified that both isomers sit alongside their dissociation products C 2 H 4 + CO , at the bottom of the potential energy surface with energies very close to each another. The most striking feature from preliminary calculations is that the relative energy difference between methylketene and trans-acrolein ( Δ E 0.8–3.4 kJ/mol) changes with respect to choice of reference geometry, correlation treatment, and basis set expansion. While the thermodynamic profile favours trans-acrolein as the lowest-lying isomer, it is difficult to estimate the quality of the currently available results, which are determined with a variety of methods. At higher levels of theory together with reducing basis set incompleteness methylketene might be the true minimum on the C 3 H 4 O potential energy surface. In addition to their being almost isoenergetic, much interest in these C 3 H 4 O isomers stems from an astrochemical standpoint. The discovery of both cis and trans-acrolein in the interstellar medium has led to speculation that methylketene may also be present. While thermodynamically methylketene proves to be a good candidate for extra-terrestrial observation, as yet there have been no reports of its detection. To this end, by applying the best correlation treatment that has been used for this species, using by far the most elaborate and accurate basis sets (up to sextuple-zeta quality), we establish the relative ordering and energetic difference between methylketene and trans-acrolein to within chemical accuracy (within 1 kJ/mol). We find the inclusion of zero-point energy, explicit core-correlation, and augmentation with diffuse functions have significant effects in determining the relative energy difference of the isomers, and assignment of the C 3 H 4 O global minimum. [ABSTRACT FROM AUTHOR]

The total atomization energy of BF3 was accurately obtained ab initio in a comprehensive study employing coupled cluster techniques to compute the heat of formation of boron atom. Basis sets up to (8s7p6df4g3h2i) quality were analyzed and an extrapolation for further basis set incompleteness was used. Inner-shell correlation, anharmonicity in the zero-point energy and atomic spin-orbit splitting were all considered. The best heat of formation for gaseous boron at 0 K was 136.0 kcal/mol.

The Ã1A1 ← X...1A1 band system of the propargyl cation (H2C3H+) is measured over the 230-270 nm range by photodissociation of mass-selected H2C3H+-Ne and H2C3H+-N2 complexes in a tandem mass spectrometer. The band origin occurs at 37 618 cm-1 for H2C3H+-Ne and 37703 cm-1 for H2C3H+-N2. Ground and excited state ab initio calculations for H2C3H+ using the MCSCF and coupled-cluster (CC) response methods show that the ion has C2v symmetry in the ground X...1A1 and excited Ã1A1 states and that the strong vibronic progression with a spacing of 630 cm-1 is due to the C-C stretch vibrational mode, v5. [ABSTRACT FROM AUTHOR]

Aluminum reacted with certain carbon atoms to yield dialuminum acetylide, while certain energized aluminum atoms reacted with acetylene to produce aluminum acetylide. These linear molecules were similar to acetylene as they contained similar carbon triple bonds. A pi complex was obtained when thermal Al atoms were added to acetylene.

Fungal azaphilones are a broad class of naturally‐occurring pigments with diverse applications. Among the azaphilone pigments, mitorubrins are well recognized for their antiviral, antibacterial, antifungal, antiprotozoal, antidiabetic, and antiaging activities in addition to their well‐known yellow‐orange color. This makes these pigments interesting candidates for use in foods, as cosmetics, and as medicines. In particular, if it is desired to modify the properties of mitorubrin‐based pigments, for example by derivatization, it is essential to have an understanding of the electronic spectra of the parent molecules. We have therefore undertaken a computational study of a series of mitorubrins, comparing our computed results with experimental UV/visible spectra. Both density‐functional theory (DFT) and coupled‐cluster (CC2) methods have been used, and in general, the results are in very good agreement with observation. In order to provide a simple and useful picture of the spectra we analyze the stronger transitions in terms of natural transition orbitals (NTOs). [ABSTRACT FROM AUTHOR]

In this work, the resolution of the identity (RI) approximation is developed for the calculation of the electron-electron spin-spin coupling (SSC) interaction that is a central component of the zero-field splitting (ZFS) term in the effective spin Hamiltonian. The approximated integrals are then used in large-scale multireference configuration interaction treatments of the SSC interaction. The SSC contribution to the ZFS is treated using the Breit–Pauli spin-spin Hamiltonian in conjunction with first-order perturbation theory. Test calculations on a set of diatomic molecules reveal that the error of the RI approximation does not exceed 0.01 cm-1 even if standard auxiliary basis sets are used. This error of less than 1% is considered to be negligible compared to the presently achievable accuracy of the SSC calculations relative to experimental data. The present development allows the correlated ab initio calculation of ZFS parameters of larger systems such as linear polyenes and linear polyacenes. The basis set convergence of the calculated ZFS values was investigated, and the effect of electronic correlation on the calculated ZFS parameters is discussed. [ABSTRACT FROM AUTHOR]

We present the implementation of two-electron spin-spin coupling as a quasidegenerate perturbative treatment of the Breit–Pauli spin-spin Hamiltonian. The evaluation is based on a multireference CI treatment and constitutes one of the first efforts in the calculation of this effect within a highly sophisticated consideration of both nondynamical and dynamical correlation. The extension of existing schemes for efficient calculation, in particular, of the spin-coupling elements necessitated some involved derivations, the outline of which is presented herein. Application of the program to calculations of diagonal as well as off-diagonal spin-coupling elements is illustrated with the test cases O2 and NH. [ABSTRACT FROM AUTHOR]

Basis set convergence of correlation effects on molecular atomization energies beyond the coupled cluster with singles and doubles (CCSD) approximation has been studied near the one-particle basis set limit. Quasiperturbative connected triple excitations, (T), converge more rapidly than L-3 (where L is the highest angular momentum represented in the basis set), while higher-order connected triples, T3-(T), converge more slowly—empirically, ∝L-5/2. Quasiperturbative connected quadruple excitations, (Q), converge smoothly as ∝L-3 starting with the cc-pVTZ basis set, while the cc-pVDZ basis set causes overshooting of the contribution in highly polar systems. Higher-order connected quadruples display only weak, but somewhat erratic, basis set dependence. Connected quintuple excitations converge very rapidly with the basis set, to the point where even an unpolarized double-zeta basis set yields useful numbers. In cases where fully iterative coupled cluster up to connected quintuples (CCSDTQ5) calculations are not an option, CCSDTQ(5) (i.e., coupled cluster up to connected quadruples plus a quasiperturbative connected quintuples correction) cannot be relied upon in the presence of significant nondynamical correlation, whereas CCSDTQ(5)Λ represents a viable alternative. Connected quadruples corrections to the core-valence contribution are thermochemically significant in some systems. We propose an additional variant of W4 theory [A. Karton et al., J. Chem. Phys. 125, 144108 (2006)], denoted W4.4 theory, which is shown to yield a rms deviation from experimental atomization energies (active thermochemical tables, ATcT) of only 0.05 kcal/mol for systems for which ATcT values are available. We conclude that “3σ≤=1 kJ/mol” thermochemistry is feasible with current technology, but that the more ambitious goal of ±10 cm-1 accuracy is illusory, at least for atomization energies. [ABSTRACT FROM AUTHOR]

The potential energy surfaces (PESs) and S1-T1 spin-orbit coupling matrix element (SOCME) surfaces are investigated for the toluene–X weakly bound clusters (X=Ne, Ar, and Kr). Calculations of the vibrational wave functions using a one-dimensional stretch model are presented and used to determine vibrationally averaged values of the SOCMEs. Our ab initio theoretical results compare well with intersystem crossing rates derived from recent experimental fluorescence lifetime data [Doyle et al., J. Chem. Phys. 122, 194315 (2005)]. Vibrational averaging is shown to change the absolute magnitude of the calculated SOCMEs, but the ratio between them remains very similar to that of the single-point values calculated at the minima of the PESs. [ABSTRACT FROM AUTHOR]

In this work Gaussian-type Geminals (GTGs) are applied in local second-order Mo\ller-Plesset perturbation theory to improve the basis set convergence. Our implementation is based on the weak orthogonality functional of Szalewicz et al., [Chem. Phys. Lett. 91, 169 (1982); J. Chem. Phys. 78, 1420 (1983)] and a newly developed program for calculating the necessary many-electron integrals. The local approximations together with GTGs in the treatment of the correlation energy are introduced and tested. First results for correlation energies of H2O, CH4, CO, C2H2, C2H4, H2CO, and N2H4 as well as some reaction and activation energies are presented. More than 97% of the valence-shell correlation energy is recovered using aug-cc-pVDZ basis sets and six GTGs per electron pair. The results are compared with conventional calculations using correlation-consistent basis sets as well as with MP2-R12 results. [ABSTRACT FROM AUTHOR]

Highly accurate ab initio computations of the molecular structure and properties, torsional potential energy function, and harmonic force field of disilane and ethane have been carried out. Equilibrium parameters as well as vibrational corrections have been evaluated. In addition, for these systems a vibrational averaging procedure has been employed for calculating the dipole moment of molecules which have no permanent dipole moment, i.e., SiH3SiD3 and CH3CD3. The molecular and spectroscopic properties calculated for ethane and its isotopomers provide a calibration against known experimental data, allowing us to estimate the reliability of our computed results for disilane for which there is much less experimental data. The goal of the present study is to predict the molecular parameters, with estimated uncertainties, that determine the microwave spectrum of SiH3SiD3. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]