Methyl cation affinities of rare gases and nitrogen and the heat of formation of diazomethane

The methyl cation affinities of the rare gases have been calculated at 0 and 298 K by using coupled cluster theory including noniterative, quasiperturbative triple excitations with the new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. To achieve near chemical accuracy (+/-1 kcal/mol) in the thermodynamic properties, we add to the estimated complete basis set valence binding energies, based on frozen core coupled cluster theory energies, two corrections: (1) a core/valence correction and (2) a scalar relativistic correction. Vibrational zero-point energies were computed at the coupled cluster level of theory at the CCSD(T)/aug-cc-pVDZ level. The calculated rare gas methyl cation affinities (MCA in kcal/mol) at 298 K are the following: MCA(He) = 1.7, MCA(Ne) = 2.5, MCA(Ar) = 16.9, MCA(Kr) = 25.5, and MCA(Xe) = 36.6. Because of the importance of the MCA(N(2)) in the experimental measurements of the MCA scale, we calculated a number of quantities associated with CH(3)N(2)(+) and CH(2)N(2). The calculated values for diazomethane at 298 K are: DeltaH(f)(CH(2)N(2)) = 65.3 kcal/mol, PA(CH(2)N(2)) = 211.9 kcal/mol, and MCA(N(2)) = 43.2 kcal/mol.     

Methyl propionate radical cation

Examination of the reactions of the long-lived (>0.5-s) radical cations of CD₃CH₂COOCH₃ and CH₃CH₂COOCD₃ indicates that the long-lived, nondecomposing methyl propionate radical cation CH₃CH₂C(O)OCH ₃ ^+· isomerizes to its enol form CH₃CH=C(OH)OCH ₃ ^+· (ΔH _{isomerization} ? ?32 kcal/mol) via two different pathways in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. A 1,4-shift of a β-hydrogen of the acid moiety to the carbonyl oxygen yields the distonic ion ^·CH₂CH₂C⁺ (OH)OCH₃ that then rearranges to CH₃CH=C(OH)OCH ₃ ^+· probably by consecutive 1,5- and 1,4-hydrogen shifts. This process is in competition with a 1,4-hydrogen transfer from the alcohol moiety to form another distonic ion, CH₃CH₂C⁺(OH)OCH ₂ ^· , that can undergo a 1,4-hydrogen shift to form CH₃CH=C(OH)OCH ₃ ^+· . Ab initio molecular orbital calculations carried out at the UMP2/6-31G** + ZPVE level of theory show that the two distonic ions lie more than 16 kcal/mol lower in energy than CH₃CH₂C(O)OCH ₃ ^+· . Hence, the first step of both rearrangement processes has a great driving force. The 1,4-hydrogen shift that involves the acid moiety is 3 kcal/mol more exothermic (ΔH _{isomerization}=?16 kcal/mol) and is associated with a 4-kcal/mol lower barrier (10 kcal/mol) than the shift that involves the alcohol moiety. Indeed, experimental findings suggest that the hydrogen shift from the acid moiety is likely to be the favored channel.     

Binding affinities of commonly employed sensitizers of viral inactivation

Methylene blue (MB), riboflavin (RB) and psoralen sensitizers (4' aminomethyl-4,5',8-trimethylpsoralen [AMT] and derivatives) are under study as sensitizers of viral inactivation of blood products such as plasma proteins, platelets and red cells, all of which lack genomic nucleic acid. To predict where these sensitizers accumulate in viruses and in cells, their relative affinities for calf thymus DNA, neutral and negatively charged phospholipids and albumin were determined by dialysis. MB has a strong affinity for nucleic acid and negatively charged phospholipid, but little affinity for albumin or neutral phospholipid. RB has modest affinity for nucleic acid and little affinity for albumin or either phospholipid. AMT has substantial affinity for nucleic acid, neutral and negatively charged phospholipids and albumin. Neither AMT nor RB binds to poly G, although MB has some affinity for this polymer. Evidence of association of RB with guanosine monophosphate, adenosine monophosphate and tryptophan methylester hydrochloride in PBS buffer in the presence and absence of formamide was obtained from nonlinear Stern-Volmer plots and shifts in the ground state absorption spectrum of RB.     

Gas-Phase studies of valinomycin-alkali metal cation complexes: attachment rates and cation affinities

Reactions of laser-desorbed Na⁺, K⁺, Rb⁺, and Cs⁺ with thermally vaporized valinomycin generate metal-ligand complexes in a Fourier transform ion cyclotron resonance trapping cell, proving that complexes can form via gas-phase ion-molecule reactions. Although desorption of intact pre-formed complexes cannot be ruled out, this route appears minor. Relative rate constants for the complexation reactions show strong dependence on the charge densities of the cations. Competition experiments between valinomycin and the synthetic ionophores 18-crown-6 (18C6) and [2.2.2]-cryptand ([2.2.2]) show that valinomycin has a higher intrinsic alkali metal cation affinity than either 18C6 or [2.2.2], in contrast to the complex formation constants observed in methanol, where K+ affinities are in the order [2.2.2] > 18C6 > valinomycin.     

Steric effects on alkyl cation affinities of maingroup-element hydrides

We have carried out an extensive exploration of gas‐phase alkyl cation affinities (ACA) of archetypal anionic and neutral bases across the periodic system using zeroth order regular approximation‐relativistic density functional theory at BP86/QZ4P//BP86/TZ2P. ACA values were computed for the methyl, ethyl, i‐propyl and t‐butyl cations and compared with the corresponding proton affinities (PA). One purpose of this work is to provide an intrinsically consistent set of values of the 298 K ACA of all anionic (XH) and neutral bases (XH_n) constituted by maingroup‐element hydrides of groups 14–17 and the noble gases (group 18) along the periods 1–6. Another purpose is to determine and rationalize the trend in affinity for a cation as the latter varies from proton to t‐butyl cation. This undertaking is supported by quantitative bond energy decomposition analyses. Correlations are established between PA and ACA values. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

Nonuniversality of commonly used correlation-energy density functionals

The correlation energies of the helium isoelectronic sequence and of Hooke's atom isoelectronic sequence have been evaluated using an assortment of local, gradient, and metagradient density functionals. The results are compared with the exact correlation energies, showing that while several of the more recent density functionals reproduce the exact correlation energies of the helium isoelectronic sequence rather closely, none is satisfactory for Hooke's atom isoelectronic sequence. It is argued that the uniformly acceptable results for the helium sequence can be explained through simple scaling arguments that do not hold for Hooke's atom sequence, so that the latter system provides a more sensitive testing ground for approximate density functionals. This state of affairs calls for further effort towards formulating correlation-energy density functionals that would be truly universal at least for spherically symmetric two-fermion systems.     

Assessment of theoretical methods for the calculation of methyl cation affinities

The methyl cation affinity (MCA; 298 K) of a variety of neutral and anionic bases has been examined computationally with a wide variety of theoretical methods. These include high-level composite procedures such as W1, G3, G3B3, and G2, conventional ab initio methods such as CCSD(T) and MP2, as well as a selection of density functional theory (DFT) methods. Experimental results for a variety of small model systems are well reproduced with practically all these methods, and the performance of DFT based methods are far superior in comparison to their MP2 analogs for these small models. For larger model, systems including motifs frequently encountered in organocatalysts, the performance deteriorates somewhat for DFT methods, while it improves significantly for MP2, rendering the former methods unreliable for common organic bases. Thus, MP2 calculations performed in combination with basis sets such as 6-31+G(2d, p) or larger, appear to offer a practical and reliable approach to compute MCAs of organic bases.     

Methyl substituent effects in radical cation Diels-Alder reactions

The effect of methyl substitution on the Diels-Alder radical cation reaction was studied using B3LYP with a 6-31G* basis set. Five separate pathways, one concerted and four stepwise, were examined for each possible position of a methyl substituent. None of the concerted transition structures could be located without symmetry constraints, and all but one of the structures obtained under these conditions were destabilized by a second-order Jahn-Teller distortion. A concerted pathway with simultaneous bond formation at C1 and C4 is therefore excluded. Stepwise pathways that had the methyl group either on a carbon involved in the initial bond formation or in a position where it could not stabilize the radical/cation were a few kcal/mol above alternate pathways. High transition state energies for the formation of vinylcyclobutane derivatives cause it to be a minor product in general. The pathway that proceeds through an anti-intermediate is the most favored, while the pathways forming the gauche-out intermediate that converts to the anti-intermediate is also strongly represented. Both of the major pathways lead directly to the formation of the methylcyclohexene product.     

Proton and sodium cation affinities of harpagide: a computational study

The aim of this work was to estimate the proton and sodium cation affinities of harpagide (Har), an iridoid glycoside responsible for the antiinflammatory properties of the medicinal plant Harpagophytum. Monte Carlo conformational searches were performed at the semiempirical AM1 level to determine the most stable conformers for harpagide and its protonated and Na+-cationized forms. The 10 oxygen atoms of the molecule were considered as possible protonation and cationization sites. Geometry optimizations were then refined at the DFT B3LYP/6-31G level from the geometries of the most stable conformers found. Final energetics were obtained at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G level. The proton and sodium ion affinities of harpagide have been estimated at 223.5 and 66.0 kcal/mol, respectively. Since harpagide mainly provides HarNa+ ions in electrospray experiments, the DeltarG298 associated with the reaction of proton/sodium exchange between Har and methanol, MeOHNa+ + HarH+ --> MeOH2+ + HarNa+ (1), has been calculated; it has been estimated to be 1.9 kcal/mol. Complexing a methanol molecule to each reagent and product of reaction 1 makes the reaction become exothermic by 1.7 kcal/mol. These values are in the limit of the accuracy of the method and do not allow us to conclude definitely whether the reaction is endo- or exothermic, but, according to these very small values, the cation exchange reaction is expected to proceed easily in the final stages of the ion desolvation process.     

Absolute potassium cation affinities (PCAs) in the gas phase

The potassium cation affinities (PCAs) of 136 ligands (20 classes) in the gas phase were established by hybrid density functional theory calculations (B3-LYP with the 6-311+G(3df,2p) basis set). For these 136 ligands, 70 experimental values are available for comparison. Except for five specific PCA values-those of phenylalanine, cytosine, guanine, adenine (kinetic-method measurement), and Me(2)SO (by high-pressure mass spectrometric equilibrium measurement)-our theoretical estimates and the experimental affinities are in excellent agreement (mean absolute deviation (MAD) of 4.5 kJ mol(-1)). Comparisons with previously reported theoretical PCAs are also made. The effect of substituents on the modes of binding and the PCAs of unsubstituted parent ligands are discussed. Linear relations between Li+/Na+ and K+ affinities suggest that for the wide range of ligands studied here, the nature of binding between the cations and a given ligand is similar, and this allows the estimation of PCAs from known Li+ and/or Na+ affinities. Furthermore, empirical equations relating the PCAs of ligands with their dipole moments, polarizabilities (or molecular weights), and the number of binding sites were established. Such equations offer a simple method for estimating the PCAs of ligands not included in the present study.     

Molar extinction coefficients of some commonly used solvents

Molar extinction coefficients of some commonly used solvents (ethanol (C₂H₅OH), methanol (CH₃OH), propanol (C₃H₇OH), butanol (C₄H₉OH), water (H₂O), toluene (C₇H₈), benzene (C₆H₆), carbontetrachloride (CCl₄), acetonitrile (C₄H₃N), chlorobenzene (C₆H₅Cl), diethylether (C₄H₁₀O) and dioxane (C₄H₈O₂)) have been determined by a well-collimated narrow beam transmission geometry at 279, 356, 662, 1173, 1252 and 1332 keV γ rays. The total γ ray interaction cross sections of these solvents have also been determined. A good agreement has been obtained between the experimental results with the theoretical values evaluated through XCOM calculations.     

Photodecomposition of several compounds commonly used as sunscreen agents

The photolysis reactions of three compounds commonly used as a sunscreen agents, Parsol 1789 (1-[4-(1,1-dimethylethyl)phenyl]-3-(4-methoxyphenyl)-1,3- propanedione), Oxybenzone ((2-hydroxy-4-methoxyphenyl)phenyl-methanone) and Padimate O (2-ethylhexyl-4-(dimethylamino)benzoate), were investigated to provide a chemical background to aid in the understanding of the photosensitization of the sunscreen agents. Photolysis was carried out in cyclohexane for 70–140 h using a mercury vapor lamp (450W) without excluding oxygen.

Irradation of Parsol 1789 in cyclohexane yielded tert-butylbenzene, p-tert-butylbenzoic acid and p-methoxybenzoic acid; products obtained from the combination of the sunscreen with the solvent included the cyclohexyl esters of p-methoxybenzoic acid, p-tert-butylbenzoic acid and methanoic acid; products obtained from the solvent included cyclohexanol, cyclohexanone and dicyclohexyl ether.

Irradiation of Oxybenzone in the cyclohexane for 100 h produced no detectable products by either gas or liquid chromatographic analysis. Oxybenzone was recovered unchanged and no products were observed from the photoinitiated reaction of oxygen with the solvent.

Irradiation of Padimate O in cyclohexane yielded the ethylhexyl esters of p-aminobenzoic acid, p-monomethylaminobenzoic acid and p-dimethylamino (o/m)-methylbenzoic acid, as well as products from the photoinitiated reaction of oxygen with the solvent.     

An overview of commonly used semiconductor nanoparticles in photocatalysis

The depletion of non-renewable resources and rise in global warming has caused great concern to humankind. With a view to use renewable source of energy and to eliminate hazardous chemical compounds from air, soil, and water, photocatalysis utilizing solar energy is becoming a rapidly expanding technology. Semiconductor nanoparticles have the ability to undergo photoinduced electron transfer to an adsorbed particle governed by the band energy positions of the semiconductor and the redox potential of the adsorbate. A brief overview of metal oxides and sulphides that can act as sensitizers for light-induced redox processes due to their electronic structure is presented here.     

Experimental validation of Gaussian-3 lithium cation affinities of amides: implications for the gas-phase lithium cation basicity scale.

Using a refined Gaussian-3 (G3) protocol, the highest level of ab initio calculations reported so far, we have established the Li+ cation binding enthalpy (affinity) at 0 K (in kJ mol-1) for formamide (195.7), N-methylformamide (209.2), N,N'-dimethylformamide (220.0), acetamide (211.7), N-methylacetamide (222.5), and N,N'-dimethylacetamide (230.1), with an estimated maximum uncertainty of +/-8 kJ mol-1. With these six theoretical lithium cation binding affinities as reference values, the absolute Li+ affinities of imidazole and dimethoxyethane were determined by the extended kinetic method, and by adopting the statistical data treatment protocol recently proposed by Armentrout. The Li+ affinities obtained for these two ligands are in good agreement (within 6 kJ mol-1) with recent values determined by the threshold collision-induced dissociation method, and consistent with the Li+ basicity values first reported by Taft and co-workers in 1990. Our study confirms that the previously suggested, and recently implemented, downward revision of Taft's original basicity scale by 10.9 kJ mol-1 is justified for ligands with revised basicities less than 151 kJ mol-1. However, for selected ligands with Li+ basicities greater than 151 kJ mol-1, including some of the six amides studied in this work, the reported discrepancy between theoretical and experimental estimates in the revised Li+ basicity scale of Burk et al. is likely to arise from experimental uncertainties.     

Retention-indices on OV-1 of approximately 170 commonly used pesticides

A screening program is reported for approximately 170 pesticides which are regularly encountered in toxicological analysis (organophosphates, carbamates, triazines, halogenated carbohydrates, phenoxycarbonic acid derivatives, and others). It is based on the retention index according to Kovats. Temperature dependence of the retention index and derivatization procedures are also studied.Dedicated in honour to Prof. Dr. Wolfgang Arnold (Hamburg) on the occasion of his 75th birthday.We are indebted to PD Dr. Hans H. Maurer (Homburg/Saar) for stimulating discussions     

Quality of correlating functions generated from commonly used basis sets

Tests have been performed on the quality of correlating functions generated from commonly used Gaussian basis sets, such as the 4-31G and MIDI-4 sets. The atoms tested were carbon, nitrogen, and oxygen. Self-consistent field and configuration interaction (CI) calculations were performed for the ground and lower excited states of neutral atoms as well as for positive and negative ions, using the original sets. Next, after adding (1) one d, and (2) two d and one f primitive Gaussian-type functions (GTFs) to the original sets, the CI calculations were repeated. In order to investigate the quality of the correlating orbitals generated from the GTF sets, parallel calculations to those for the GTF sets were carried out with an extended set of Slater-type functions. It was found that the excitation energies change in a stepwise manner as the basis sets changed from the original sets to the original set + 1d and the original set +2d1f. The improvements in excitation energies and ionization energies were almost independent of the original sets and were found to be strongly dependent on the augmented correlation functions. © 1996 by John Wiley & Sons, Inc.     

Proton and lithium cation affinities calculated with floating orbital geometry optimization (FOGO)

The FOGO method is used to calculate proton affinities and lithium cation affinities. The molecules of primary interest in this study are the methyl-substituted amines. In addition, the lithium cation affinity of HF, H₂O, CH₃OH, H₂CO, and HCN are calculated for comparison. Geometries of all species are fully optimized with a double-zeta (DZ) basis set, including polarization on hydrogen and the first-row elements by floating orbitals. Comparison with experimental values demonstrates that structural data and proton affinities resulting from this type of ab initio calculation are of chemical accuracy. The lithium cation affinities are also reasonably well reproduced, but the small experimental differences are not within the accuracy, which can be expected from this type of calculation.     

Generalized anomeric effect on N-alkyl-amino cation affinities: a G2(+)M investigation

The gas-phase N-alkyl-amino-cation affinities (NAACA) of archetypal anionic main-group element hydrides across the Periodic Table have been investigated by means of a modified G2(+) scheme. The reactions studied include R(2)NB → R(2)N(+) + B(-) (R = H, Me; B = XH(n), n = 0-3; X = F, Cl, Br, O, S, Se, N, P, As, C, Si, Ge). Our calculations indicate that the reasonable linear correlations between NAACA and proton affinities (PA) only exist within the Period 2 anions, including H(3)C(-), H(2)N(-), HO(-), and F(-), or the anions within Periods 3-4 in the Periodic Table, which is significantly different from the alkyl cation affinities, where there is a reasonable correlation between the computed alkyl cation affinity and PA values of the set of anionic main-group element hydrides. The interesting differences can be ascribed to the generalized anomeric effect induced by n(N) → σ*(X-H) negative hyperconjugation found in R(2)NXH(n), with central atom X belonging to Groups 14-16 (X = O, S, Se, N, P, As, C, Si, Ge).     

tert-Butyl cation affinities of maingroup-element hydrides: effect of methyl substituents at the protophilic center

We have conducted an extensive computational exploration of how the gas-phase tert-butyl cation affinities (t-BCA) of archetypal anionic and neutral bases across the periodic system are affected by stepwise replacement of all hydrogen atoms at the protophilic center with methyl substituents. This study was conducted using zeroth-order regular approximation relativistic density functional theory (DFT) at the BP86/QZ4P//BP86/TZ2P level. The trends are interpreted in terms of the steric effects of methyl substituents at the protophilic center of the anionic (Me(m)XH(n-1-m)(-)) and neutral bases (Me(m)XH(n-m)). Besides insight, this work also provides an intrinsically consistent set of values of the 298 K tert-butyl cation affinities of all (partially) methyl-substituted or unsubstituted bases constituted by maingroup-element hydrides of groups 14-16 in anionic cases (Me(m)XH(n-1-m)(-)) and groups 15-17 in neutral ones (Me(m)XH(n-m)) along periods 2-6. The effect of long-range dispersion (van der Waals) interactions was estimated through dispersion-corrected density functional theory (DFT-D3) at the BP86-D3/QZ4P//BP86/TZ2P level.