Tamás Veszprémi

Carbenes in ionic liquids

The chemistry of 1,3-dialkylimidazolium-based ionic liquids (ILs) can easily be linked to that of N-heterocyclic carbenes (NHCs) in the presence of sufficiently basic counteranions. B3LYP/6-31+G*, B3LYP/aug-cc-pVTZ and MP2/6-311+G** studies show that increasing the basicity of the anionic component the relative stability of the ion pair and that of the hydrogen bonded complex of the corresponding free acid and NHC itself can be shifted toward the formation of NHC. In the case of the acetate anion, the ion pair and the NHC-acetic acid complex have similar stability. Photoelectron spectroscopic studies show that the vapor of EMIM-acetate is dominated by the NHC-acetic acid complex. The mass spectrum of the same compound shows the presence of both acetic acid and 1-ethyl-3-methylimidazolium-2-ylidene, in agreement with the low pressure during the MS experiment, which facilitates dissociation. The possibility of systematic and simple variation of the NHC content of the ILs facilitates the extension of carbene chemistry in ionic liquids.

About the aromaticity of five-membered heterocycles

Abstract The aromaticity of five-membered heterocycles with one and two heteroatoms (O, S, NH, N and P) has been investigated, using energetic and structural criteria. It has been shown that to obtain a correct order for the aromaticity, a nonhomodesmic reaction (SEH) is sufficient, provided it compares the energy of the ring to that of the conjugated building blocks in the ring. Contrary to general expectations, superhomodesmic reactions seem less reliable owing to unpredictable steric interactions in the conjugated reference molecules. The stabilization shows a considerable increase if calculated at the MP2 versus HF level of theory in all reactions. The aromatic stabilization in the ring is mainly determined by the “first heteroatom” which provides two electrons for the π-system (O, S or NH). The effect of the second heteroatom is much smaller. In agreement with earlier observations, N as a second heteroatom increases the stabilization in the SEH reaction. In the case of P, stabilization has again been observed, but only at the MP2 level. An aromaticity index has been defined based on the double bond characters of the peripheral bonds in the ring. According to this geometry aromaticity index ( I ), there is no apparent increase in aromaticity for the phosphorus compounds; thus, I can be related to the decrease in ring strain, attributable to the small bonding angle about phosphorus.

Investigation of heterocyclic compounds containing a PC or AsC bond by ultraviolet photoelectron spectroscopy

Abstract The He(I) ultraviolet photoelectron spectra of benzene-fused heterocycles containing AsC and PC double bond have been recorded, and interpret MNDO and CNDO/S quantum-chemical calculation carried out to assist interpretation of the data. The analysis of the spectra indicates that there is a strong interaction between the EC double bond and the π-system of the molecule. In contrast the conjugative effect of the phosphorous or arsenic lone pair gives rise to completely different spectra for 1,3-benzazaphospholes or -arsoles substituted in 1 or 3 positions.

Stabilized carbenes do not dimerize

Dimerization Gibbs free energies were computed for several carbenes. At the B3LYP/6-311+G(2D) level the values are smaller than at MP2/6-311+G(2D) by about 10 kcal mol−1, the DFT results being in accord with experimental findings. The smallest dimerization energy was obtained for those compounds, which have already been synthesized (with proper substituent groups). The dimerization energy shows an excellent linear correlation with the stabilization obtained in an isodesmic reaction, giving the possibility to estimate the stability of any new nucleophilic carbene by a simple computational procedure. In this estimate, however, only electronic factors are considered, steric effects (the use of bulky protecting groups) can give additional stabilization.

Accurate interaction energies at density functional theory level by means of an efficient dispersion correction.

This paper presents an approach for obtaining accurate interaction energies at the density functional theory level for systems where dispersion interactions are important. This approach combines Becke and Johnsons [J. Chem. Phys. 127, 154108 (2007)] method for the evaluation of dispersion energy corrections and a Hirshfeld method for partitioning of molecular polarizability tensors into atomic contributions. Due to the availability of atomic polarizability tensors, the method is extended to incorporate anisotropic contributions, which prove to be important for complexes of lower symmetry. The method is validated for a set of 18 complexes, for which interaction energies were obtained with the B3LYP, PBE, and TPSS functionals combined with the aug-cc-pVTZ basis set and compared with the values obtained at the CCSD(T) level extrapolated to a complete basis set limit. It is shown that very good quality interaction energies can be obtained by the proposed method for each of the examined functionals, the overall performance of the TPSS functional being the best, which with a slope of 1.00 in the linear regression equation and a constant term of only 0.1 kcal/mol allows to obtain accurate interaction energies without any need of a damping function for complexes close to their exact equilibrium geometry.

Assignment of photoelectron spectra of the acenes with the help of modified CNDO/S calculations

Abstract The CNDO/S method was modified to yield results in agreement with UPS and Penning ionization electron spectroscopy experiments. With the help of the calculations we propose a number of new assignments of the σ bands for some of the acenes.

The Cu7Sc Cluster is a Stable σ‐Aromatic Seven‐Membered Ring

Density functional theory calculations demonstrate that the global minimum of the Cu(7)Sc potential energy surface is a seven-membered ring of copper atoms with scandium in its center, yielding a planar D(7) (h) structure. Nucleus-independent chemical shifts [NICS(1)(zz) and NICS(2)(zz)] show that this cluster has aromatic character, which is consistent with the number of 4s electrons of copper and scandium plus the 3d electrons of scandium satisfying Hückels rule. According to a canonical MO decomposition of NICS(1)(zz) and NICS(2)(zz), the MOs consisting of the 4s atomic orbitals are mainly responsible for the aromatic behavior of the cluster. The electron localizability indicator (ELI-D) and its canonical MO decomposition (partial ELI-D) suggest that a localized basin is formed in Cu(7)Sc by the copper atoms whereas the two circular localized domains are situated below and above the ring. The planar Cu(7)Sc cluster can thus be considered as a sigma-aromatic species. These findings agree with the phenomenological shell model.

The use of atomic intrinsic polarizabilities in the evaluation of the dispersion energy

The recent approach presented by Becke and Johnson [J. Chem. Phys. 122, 154104 (2005); 123, 024101 (2005); 123, 154101 (2005); 124, 174104 (2006); 124, 014104 (2006)] for the evaluation of dispersion interactions based on the properties of the exchange-hole dipole moment is combined with a Hirshfeld-type partitioning for the molecular polarizabilities into atomic contributions, recently presented by some of the present authors [A. Krishtal et al., J. Chem. Phys. 125, 034312 (2006)]. The results on a series of nine dimers, involving neon, methane, ethene, acetylene, benzene, and CO(2), taken at their equilibrium geometry, indicate that when the C(6), C(8), and C(10) terms are taken into account, the resulting dispersion energies can be obtained deviating 3% or 8% from high level literature data [E. R. Johnson and A. D. Becke, J. Chem. Phys. 124, 174104 (2006)], without the use of a damping function, the only outlier being the parallel face-to-face benzene dimer.

A study of some gas-phase lanthanide plus oxidant chemiionization reactions with chemielectron spectroscopy

Abstract An investigation has been made of some gas-phase lanthanide plus oxidant chemiionization reactions with electron spectroscopy. The oxidants used were O 2 (X 3 Σ g − ), O 2 (a 1 Δ g ) and O( 3 P) and the lanthanide metals (Ln) studied were Pr, Nd, Sm, Eu and Gd. In the case of europium, no ions or electrons were observed for the Eu+O 2 (X 3 Σ g − ), Eu+O 2 (a 1 Δ g ) and Eu+O( 3 P) reactions. For the other four lanthanides, studies of the equivalent metal oxidation reactions showed that when each metal is reacted with discharged oxygen, the dominant chemiionization channel is the associative ionization reaction Ln+O( 3 P) → LnO + +e − . In each case, this conclusion was supported by comparison of available standard enthalpies for possible chemiionization reactions with the observed high kinetic energy offset of the observed chemielectron band, as well as by mass analysis of the ions produced. Results of earlier studies made by electron spectroscopy of the reactions of Ce and La with O 2 (X 3 Σ g − ), O 2 (a 1 Δ g ) and O( 3 P) are discussed in the light of results obtained in this work.

Substituent effect of second row elements on silyl centers

Abstract The entire set of single- and double-bonded second row element-substituted silicon compounds, silyl radicals and silylenes, along with their parent silicon hydrides, SiHn (n = 2–4), have been investigated by using ab initio methods. All structural parameters were optimized by use of the 6–31 G∗ basis sets at the HF and MP2 levels of theory. The main structural features of the low-valent silicon species do not differ significantly from those of the first row element-substituted counterparts. Shortening of the SiR (R = substituent) bond (relative to the unsaturated derivatives) is observable with substituents from the left side of the periodic table if there is an odd electron on the silyl center (silyl radical or triplet silylene), while substituents from the right side of the periodic table cause shortening of the otherwise longer (cf. SiH2 and SiH4) Siz.sbnd;R bonds in the case of singlet silylenes. Stabilization energies (relative to the saturated silanes) due to the substitution were evaluated by using isodesmic reactions. It has been shown that, while silyl radicals and triplet silylenes are stabilized by substituents having empty p orbitals (being capable of delocalizing the odd electron on the silicon center), singlet silylenes are stabilized by substituents having unshared electron pairs (being capable of delocalizing to the empty p orbital of the silylene unit). Rotational barriers about the Siz.sbnd;R bonds give similar va the isodesmic stabilization energies. Stabilization caused by the second row elements is not less than that of first row substituents,depends instead on the electron donor/acceptor properties of the substituting group. In certain cases (H2AlSiH2, HSiPH2) the delocalization stabilization energy is significantly reduced (compared to the expectations based on bond shortening and electronegativity of the substituting group), as the inversion barriers (at the silyl and phosphine centers respectively) should be surmounted by the delocalization stabilization. The strong electron-acceptor character of singlet silylene can be rationalized by the reduction of the phosphine inversion barrier (about 35 kcal mol−1 in PH3) to 0.2 kcal mol−1 at the MP2/6–31G∗//MP2/6–31G∗ level of theory (0.35 and 0.4 kcal mol−1 at the MP4/6–31G∗//MP2/6–31G∗ and MP2/6–31G∗∗//MP2/6–311G∗∗ levels of theory respectively). This reduction of the inversion barrier is even larger than that reported for BH2 substituent.