Local orbitals for the study of the π→π* excitation in polyenes

Localized orbitals have recently been employed in large ab initio calculations, but their use has generally been restricted to ground‐state problems. In this work, we analyze the molecular orbitals of the excited states, optimized with a recently proposed local procedure. This method produces local orbitals of the CAS–SCF type, which permits its application to the study of excited states. In particular, we focus on the π→π* triplet excited state in polyenes, calculated using a 2/2 CAS space which includes two electrons in one π and one π* orbitals. In small polyenes, these two singly occupied active orbitals are delocalized all along the molecule. The extent of the delocalization is analyzed by studying polyenes of increasing size. Different polyenes have been studied, going from C₁₄H₁₆ to the C₇₀H₇₂ polyene. The relation of the π→π* excitation with the cation and anion systems is also discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

A CASSCF study on the lowest π→π* excitation of urocanic acid

The photochemical cis/trans isomerization of urocanic acid (UCA, (E)‐3‐(1′H‐imidazol‐4′‐yl)propenoic acid) was investigated using complete active space SCF (CASSCF) ab initio calculations. The singlet ground state and the triplet and the singlet manifolds of the lowest‐lying π→π* (HOMO→LUMO) excitation of the neutral and the anionic UCA were calculated using the 6‐31G* and the 6‐31+G* basis sets, respectively. The torsional barrier of the double bond of the propenoic acid moiety in UCA is observed to be considerably lower in the T₁ and S₁ excited states of the neutral UCA and in the T₁ but not in the S₁ excited state of the anionic UCA, as compared to the S₀ state of the respective protonation form. The cis‐isomer of both the neutral and the anionic UCA is lower in energy than the trans‐isomer in the S₀, T₁, and S₁ states. This energy difference is larger in the excited states than in the ground state, probably due to strengthening of the intramolecular hydrogen bond of cis‐UCA as the molecule is excited. The results of the calculations, interpreted in terms of the idea that UCA is deprotonated upon electronic excitation, led to construction of a new model for the photoisomerization mechanisms of UCA. According to this model, the trans‐to‐cis isomerization proceeds via both the triplet and the singlet manifolds in the deprotonated form of UCA. This isomerization may occur in the S₀ state of the neutral UCA as well. The cis‐to‐trans isomerization is suggested to proceed only in the S₀ state of the neutral UCA. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 25–37, 1999     

Ab initio study of 4-monosubstituted pyrimidines in ground and excited n → π* states

Ab initio SCF and SCF -CI calculations have been performed to investigate substituent effects on ground- and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2- and 4-R-pyridines, with R including the π donating and σ withdrawing groups CH₃, NH₂, OH, F, and C₂H₃ and the σ and π electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a π donating group leads to an increase in the n→π* transition energy of 4-R-pyrimidines, while the π withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy differences and virtual excitation energies tend to correlate with n→π* transition energies of 4-R-pyrimidines with saturated R groups, but such correlations are masked by π conjugation, n orbital interaction, and configurational mixing when the unsaturated groups C₂H₃, CHO, and CN are present. The electronic effects of a π donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a π withdrawing group are weaker when the group is bonded to pyrimidine.     

Direct observation of a 220 cm structure in the lowest π–π* absorption band in the vapour phase of trans-azobenzene

In this work we present the first direct observation of a 220 cm⁻¹ progression in the absorption spectrum of trans-azobenzene in the vapour phase. The mode at 220 cm⁻¹ is essential to explain both the electronic absorption spectrum and the Raman excitation profiles of the most intense Raman bands of trans-azobenzene in the 1000–1600 cm⁻¹ shift range. Our data in the vapour phase assure that this frequency pertains to an internal molecular mode.     

Assessment of standard force field models against high‐quality ab initio potential curves for prototypes of π–π, CH/π, and SH/π interactions

Several popular force fields, namely, CHARMM, AMBER, OPLS‐AA, and MM3, have been tested for their ability to reproduce highly accurate quantum mechanical potential energy curves for noncovalent interactions in the benzene dimer, the benzene‐CH₄ complex, and the benzene‐H₂S complex. All of the force fields are semi‐quantitatively correct, but none of them is consistently reliable quantitatively. Re‐optimization of Lennard‐Jones parameters and symmetry‐adapted perturbation theory analysis for the benzene dimer suggests that better agreement cannot be expected unless more flexible functional forms (particularly for the electrostatic contributions) are employed for the empirical force fields. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

An ab initio valence bond (VB) calculation of the π delocalization energy in borazine,B3N3H6

Valence bond (VB) calculations using a double‐zeta D95 basis set have been performed for borazine, B₃N₃H₆ and for benzene, C₆H₆ in order to determine the relative weights of individual standard Lewis structures. In the delocalized resonance scheme of borazine, the structure ( I ) with no double bonds and three lone pairs of electrons at the three nitrogen atoms is the major contributor with a structural weight of 0.17, followed by six equivalent Lewis structures with one double bond and two lone pairs at two nitrogen atoms ( II ) with weights of 0.08 each. In the case of benzene, the two Kekulé structures ( III ) contribute with structural weights of 0.15 each, followed by 12 equivalent ionic structures ( IV ) with weights of 0.03 each, followed by the three equivalent Dewar‐type structures ( V ) with structural weights of 0.02 each. The values of 54.1 and 45.8 kcal mol⁻¹ for the delocalization energies of borazine and benzene were estimated. Therefore, B₃N₃H₆ is calculated to have substantial aromatic character, similar to benzene, when we assume that the resonance energy can provide a criterion for aromaticity. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:311–315, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20095     

The effect of a positive charge on a π → π-transition

The position of the maximum for the π → π^*-transition of a bicyclic ketone containing a quaternary nitrogen atom at 3·1 Å from an :β-unsaturated ketone system is shown to depend largely upon electrostatic (repulsion) destabilization in both ground and excited states, the latter being affected more strongly.     

n→π*-Übergänge in Dicarbonylverbindungen Der Einfluss der n,n- und π*, π*-Wechselwirkung

By CNDO-CI calculations we have found that dicarbonyl compounds exhibit only two n → π* transitions in the visible or near UV. region, instead of four as expected from simpler MO-models. The dominant features of the long-wavelength electronic spectra may be characterized by the relative energy of the two n and the two lowest π* orbitals. In general we distinguish between three cases:

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Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance‐dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short‐range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O???C=O interactions.     

An ab initio calculation of the absorption cross section of water in the first absorption continuum

We present a complete ab initio study of the photodissociation of water in the first absorption band including a calculated excited-state potential and a calculated transition dipole function. The dynamical treatment is based on the IOS approximation for the rotational degree of freedom. The vibrationally coupled equations are solved exactly using polar coordinates. Agreement with the experimental absorption spectrum is excellent and even the “vibrational” structures are reproduced. A dynamical interpretation of these structures in terms of features of the à ¹B₁ surface is given.     

An ab initio study on the vibrational and electronic spectra of the molybdenum–sulfur clusters with Mo2OnS4−n(n=0–4) core

Using the ab initio method, the vibrational and electronic spectra of binuclear molybdenum clusters which contain Mo₂O_nS_4−n(n=0–4) core were investigated. The main absorption bands in the IR spectra of these clusters are assigned and compared with each other, especially for the case of the trans isomers. The electronic spectra were studied by performing the CIS calculations. The ground state and the first excited state of the clusters were discussed by using the natural bond orbital method. It is shown that the band corresponding to the longest wavelength can be assigned to three kinds of transition types. Two transitions, σ(Mo–Mo)→π*(Mo–X_t)(X=S,O) and σ(Mo–Mo)→σ*(Mo–Mo), can be seen in most cases.