Heidi M. Muchall

Ultraviolet Absorption Spectra of Substituted Phenols: A Computational Study†

Abstract Vertical excitation energies for electronic transitions from the ground state to the first two excited states of phenol, mono- and disubstituted methoxyphenols and methyl-substituted phenols have been characterized with the Time-Dependent Density Functional Theory (TD-DFT), the Complete Active Space Self-Consistent Field method (CASSCF) and the Coupled Cluster with Single and Double Excitations Equation-of-Motion approach (CCSD-EOM) to simulate and interpret experimental ultraviolet absorption spectra. While CASSCF excitation energies for the first two transitions either are grossly overestimated or exhibit a weak correlation with experimental data, both TD-DFT and CCSD-EOM perform very well, reproducing the spectral shifts of both the primary band and secondary band observed upon substitution. The conformational dependence of the calculated excitation energies is generally smaller than the shifts caused by substitution.

Photoelectron spectroscopic and computational study of the thermolysis of 1,2,3,6-tetrahydro-1,2,4,5-tetrazines

Abstract Gas phase thermolyses of 3,6-dimethyl, -diethyl and -di-n-propyl substituted 1,2,3,6-tetrahydro-1,2,4,5-tetrazines 1a–1c yielded the corresponding N-unsubstituted imines 2a–2c. The He(I) photoelectron spectra of propanimine (2b) and butanimine (2c) are presented for the first time. First ionization potentials and orbital energies of the imines as determined with B3LYP/6-31+G*//HF/6-31+G* reproduce the experimental data. Ionization bands arising from N,N′-unsubstituted diazetidines 3a–3c could not be detected in the pyrolysis spectra of 1a–1c. Calculations for the extrusion of nitrogen from tetrahydrotetrazine 1a resulted in a transition state for a [2+2+2]-cycloreversion that leads to the imine 2a as product.

Revisiting the effects of sequence and structure on the hydrogen bonding and π-stacking interactions in nucleic acids.

Calculated electron densities from PBE0/6-31+G(d,p) were analyzed with respect to the hydrogen bonding within a nucleic acid base pair and the π-stacking between sets of base pairs. From published X-ray crystallographic data, base pairs were isolated from a total of 11 DNA and RNA duplexes, and their experimental geometry was maintained throughout the analyses. Focusing solely on Watson-Crick base pairs, from the values of the electron density between interacting nuclei (at the bond critical points), we provide quantitative data on individual weak interactions. For hydrogen bonding, in addition to quantifying the scissoring effect in GC base pairs, the origin of the controversy around the relative stability of AT and AU base pairs is identified and resolved. Thus, it is illustrated how the conclusion as to their relative stability rests on the specific choice of oligonucleotides compared. For π-stacking, sequence effects for tandem AT base pairs are captured, quantified, and explained, and the greater sensitivity of GC, over AT, sequences to the rise parameter is established. The results presented here show that, from experimental geometries and their electron densities, previously determined effects of the sequence and structure of a duplex on the stabilizing interactions can be captured, quantified, and traced back to the geometry of the base pairs.

New Insights into the Use of (TD-)DFT for Geometries and Electronic Structures of Constrained π-Stacked Systems : [n.n]Paracyclophanes

We have evaluated the performance of three (TD-)DFT functionals with the 6-31+G(d,p) basis set for the reproduction of experimental geometries, vertical ionization potentials, and low excitation energies of a selection of [2.2] and [3.3]paracyclophanes. Overall, (TD-)BH&H outperforms both (TD-)B3LYP and (TD-)PBE0. Some shortcomings are shown by B3LYP for geometries and by BH&H for ionization potentials. Most notably, whereas TD-B3LYP and TD-PBE0 reproduce the wavelength for the first electronic excitation of [n.n]paracyclophanes with weakly interacting aromatic rings, neither handles the strong donor-acceptor interactions in certain substituted [n.n]paracyclophanes, and both seriously underestimate the energy for their first electronic excitation. As the former systems are in many ways similar to stacked nucleic acid bases, we recommend the use of (TD-)PBE0/6-31+G(d,p) for further studies on pi-stacking interactions in constrained systems, such as the base pairs in oligonucleotides.

The electronic structure of nitrilimines revisitedElectronic supplementary information (ESI) available: Optimized structures and absolute energies of all stationary points. See http://www.rsc.org/suppdata/cc/b4/b407302a/

A combination of density-functional theory and natural resonance theory has been used to show that a complete description of the electronic structure of nitrilimines, R(1)CNNR(2), requires four resonance structures (propargylic, allenic, 1,3-dipolar and carbenic); appropriate substituents were shown to enhance the carbene character of nitrilimines to the point where they may be considered stable carbenes.

The 2-norbornyl cation is a π-complex, not a σ-bridged, nonclassical species: an AIM study

Abstract The 2-norbornyl cation is not a nonclassical, σ -bridged species but a π -complex. This was revealed in a new atoms in molecules (AIM) study of the topology of its charge density. Similarly, there is nothing nonclassical about the 2-bicyclo[2.2.2]octyl cation.

Conformational properties and electronic structure of tetrahydrotetrazines studied by photoelectron spectroscopy and quantum chemical calculations

Abstract The photoelectron (PE) spectra of tetrahydro-1,2,3,4-tetrazines 1 and 2 and tetrahydro-1,2,4,5-tetrazines 3–5 have been recorded and their conformations have been investigated by ab initio SCF calculations. While v-tetrazine2 is planar, tetrazines 1 and 3–5 each possess two low-energy conformations, according to ab initio HF and Becke3LYP methods. Attempts to assign ionization potentials to molecular orbitals obtained by semiempirical PM3 calculations indicate that this method is not suited for the compounds studied. Best results were obtained when the ab initio hybrid method Becke3LYP of the density functional theory was employed. Two conformers of 1 and 3–5 are present in the gas phase and their PE spectra are superimposed one upon the other. For v-tetrazine1, ionizations arising from half-chair and unsymmetrical boat conformers have similar energies and cannot be separated in the PE spectrum. For s-tetrazine3, on the other hand, the spectrum clearly shows different ionizations of both half-chairs, 3ee and 3ae.

Computational studies on acyclic amidyl radicals: Π and Σ states and conformations

Abstract Restricted open and unrestricted Becke3LYP/6-31+G(d) calculations on Π and Σ states as well as equilibrium geometries of the formamidyl radical (1) and four of its dialkyl substituted derivatives 2–5 have been carried out. While all radicals studied are significantly twisted about the RN–C(O) bond and show a Π-type total spin density, the calculations confirm the special status of N-tert-butyl acetamidyl (4) that was found with EPR spectroscopy. Each of the torsional double-minimum potentials of N-methyl and N-isopropyl radicals 2, 3, and 5 shows a low barrier to interconversion for two equivalent conformers whereas 4 is situated in a steeper well with a larger twist angle which explains reported EPR 13C hyperfine splittings.

From inert to explosive, the hydrolytic reactivity of R-NSO compounds understood: a computational study.

We present a computational study on the concerted hydrolysis of several classes of N-sulfinylamines of generic formula R-N═S═O, such as the -amines themselves (R-NSO), -hydrazines (R-NH-NSO), -hydrazides (R-CO-NH-NSO) and -amides (R-CO-NSO), as these species are known to possess a wide range of hydrolytic reactivity. Two possible mechanisms of hydrolysis, with a water dimer across the S═O and N═S bonds, in the gas phase are investigated. The reactivity is discussed with respect to the electronic structures, established with the use of the quantum theory of Atoms in Molecules, Natural Bond Orbital and Natural Resonance Theory approaches. For the inert N-sulfinylhydrazines and the keto tautomers of N-sulfinylhydrazides, extended π-conjugation adds a sulfide-like resonance structure that is responsible for their insensitivity toward moisture. Activation barriers for hydrolysis, where water acts as a nucleophilic reagent, decrease with increasing positive charge on the NSO sulfur atom, a finding that might prove useful as a predictive tool in the determination of the general reactivity of an N-sulfinyl compound by experimentalists.

A general measure of conjugation in biphenyls and their radical cations

Changes in the amount of conjugation in the biphenyl system upon torsion have been investigated with quantum chemistry computations. For the unsubstituted biphenyl and its radical cation, the chang...