Rearrangements of 3-aryl-substituted cyclopropenyl anions and the gas-phase acidity of 3-(4-methylphenyl)cyclopropene.

3-(4-Methylphenyl)-3-trimethylsilylcyclopropene and 3-(4-trifluoromethylphenyl)-3-trimethylsilylcyclopropene react with fluoride ion in the gas phase to afford 6-substituted 3-indenyl anions via a spontaneous rearrangement of their corresponding cyclopropenyl anions. These isomerizations led us to reinvestigate the reported gas-phase generation of 1,2,3-triphenylcyclopropenyl anion, and contrary to the previous study, a similar rearrangement to 1,2-diphenyl-1-indenyl anion is observed. Despite the instability of 3-aryl-3-cyclopropenyl anions, we were able to measure the acidity of 3-(4-methylphenyl)cyclopropene at the allylic position (delta H(o)acid = 398.6 +/- 1.4 kcal/mol) by the DePuy kinetic method. Ab initio calculations on the structures and energies of mono- and triaryl-substituted cyclopropenyl anions also are presented.     

Determination of the gas-phase acidities of cysteine-polyalanine peptides using the extended kinetic method

We determined the gas-phase acidities of two cysteine-polyalanine peptides, HSCA3 and HSCA4, using a triple-quadrupole mass spectrometer through application of the extended kinetic method with full entropy analysis. Five halogenated carboxylic acids were used as the reference acids. The negatively charged proton-bound dimers of the deprotonated peptides with the conjugate bases of the reference acids were generated by electrospray ionization. Collision-induced dissociation (CID) experiments were carried out at three collision energies. The enthalpies of deprotonation (Delta(acid)H) of the peptides were derived according to the linear relationship between the logarithms of the CID product ion branching ratios and the differences of the gas-phase acidities. The values were determined to be Delta(acid)H(HSCA3) = 317.3 +/- 2.4 kcal/mol and Delta(acid)H (HSCA4) = 316.2 +/- 3.9 kcal/mol. Large entropy effects (Delta(DeltaS) = 13-16 cal/mol K) were observed for these systems. Combining the enthalpies of deprotonation with the entropy term yielded the apparent gas-phase acidities (Delta(acid)G(app)) of 322.1 +/- 2.4 kcal/mol (HSCA3) and 320.1 +/- 3.9 kcal/mol (HSCA4), in agreement with the results obtained from the CID-bracketing experiments. Compared with that in the isolated cysteine residue, the thiol group in HSCA3,4 has a stronger gas-phase acidity by about 20 kcal/mol. This increased acidity is likely due to the stabilization of the negatively charged thiolate group through internal solvation.     

Electronic Factors Influencing the Decarboxylation of beta-Keto Acids. A Model Enzyme Study

A theoretical study of the mechanism of decarboxylation of beta-keto acids is described. A cyclic transition structure was found with essentially complete proton transfer from the carboxylic acid to the beta-carbonyl group. The activation barrier for decarboxylation of formylacetic acid is predicted to be 28.6 kcal/mol (MP4SDTQ/6-31G//MP2/6-31G) while loss of CO(2) from its anion exhibits a barrier of only 20.6 kcal/mol (MP4SDTQ/6-31+G//MP2/6-31+G). Barrier heights of decarboxylation of malonic acid and alpha,alpha-dimethylacetoacetic acid are predicted to be 33.2 and 26.7 kcal/mol, respectively. Model enzyme studies using a thio methyl ester of malonate anion suggests that the role of malonyl-CoA is to afford a polarizable sulfur atom to stabilize the developing enolate anion in the transition structure for decarboxylation. Adjacent positively charged ammonium ions are also observed to stabilize the loss of CO(2) from a carboxylate anion by through-bond Coulombic stabilization of the transition structure.     

Rearrangements and interconversions of heteroatom-substituted isocyanates, isothiocyanates, nitrile oxides, and nitrile sulfides, RX-NCY and RY-CNX

Isocyanates and isothiocyanates of the type RX-NCY (X and Y = O or S) and the isomeric nitrile oxides and nitrile sulfides RY-CNX are highly reactive compounds. A number of potential 1,4-shifts of substituent groups of the type R-Y-CNX → R-X-N═C═Y, 1,3-shifts R-C(═Y)-N═X → R-X-N═C═Y, and 1,2-shifts R-C(═Y)-N═X → R-Y-CNX have been evaluated computationally. The results obtained for the relatively new functional MPW1K and the well-established B3LYP, together with a triple-ζ quality basis set, are very similar. The 1,3- and 1,4-halogen shifts in the title compounds are usually highly exothermic and possess low activation barriers. 1,3-Aryl shifts are feasible for for 5e → 6e (Ar-CO-NSO(2) → Ar-SO(2)-NCO) with activation barriers of less than 40 kcal/mol. Additionally, several 1,3- and 1,4-hydrogen shifts and the 1,4-methyl-shift in methoxynitrile sulfide MeO-CNS to methylsulfenyl isocyanate MeS-NCO (4c → 6c) are potentially feasible. The 1,2-shift reactions 4b → 5b (HO-NCS → H-CS-NO) and 4c → 5c (Ar-O-CNS→ Ar-CO-NS) are good candidates for experimental observation with activation energies around 30 kcal/mol.     

Characterization of Nα-Fmoc-protected dipeptide isomers by electrospray ionization tandem mass spectrometry (ESI-MS(n)): effect of protecting group on fragmentation of dipeptides

A series of positional isomeric pairs of Fmoc-protected dipeptides, Fmoc-Gly-Xxx-OY/Fmoc-Xxx-Gly-OY (Xxx=Ala, Val, Leu, Phe) and Fmoc-Ala-Xxx-OY/Fmoc-Xxx-Ala-OY (Xxx=Leu, Phe) (Fmoc=[(9-fluorenylmethyl)oxy]carbonyl) and Y=CH(3)/H), have been characterized and differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-IT-MS(n)). In contrast to the behavior of reported unprotected dipeptide isomers which mainly produce y(1)(+) and/or a(1)(+) ions, the protonated Fmoc-Xxx-Gly-OY, Fmoc-Ala-Xxx-OY and Fmoc-Xxx-Ala-OY yield significant b(1)(+) ions. These ions are formed, presumably with stable protonated aziridinone structures. However, the peptides with Gly- at the N-terminus do not form b(1)(+) ions. The [M+H](+) ions of all the peptides undergo a McLafferty-type rearrangement followed by loss of CO(2) to form [M+H-Fmoc+H](+). The MS(3) collision-induced dissociation (CID) of these ions helps distinguish the pairs of isomeric dipeptides studied in this work. Further, negative ion MS(3) CID has also been found to be useful for differentiating these isomeric peptide acids. The MS(3) of [M-H-Fmoc+H](-) of isomeric peptide acids produce c(1)(-), z(1)(-) and y(1)(-) ions. Thus the present study of Fmoc-protected peptides provides additional information on mass spectral characterization of the dipeptides and distinguishes the positional isomers.     

Heats of formation of boron hydride anions and dianions and their ammonium salts [BnHmy-][NH4+]y with y=1-2

Thermochemical parameters of the closo boron hydride BnHn2- dianions, with n=5-12, the B3H8- and B11H14- anions, and the B5H9 and B10H14 neutral species were predicted by high-level ab initio electronic structure calculations. Total atomization energies obtained from coupled-cluster CCSD(T)/complete basis set (CBS) extrapolated energies, plus additional corrections were used to predict the heats of formation of the simplest BnHmy- species in the gas phase in kcal/mol at 298 K: DeltaHf(B3H8-)=-23.1+/-1.0; DeltaHf(B5H52-)=119.4+/-1.5; DeltaHf(B6H62-)=64.1+/-1.5; and DeltaHf(B5H9)=24.1+/-1.5. The heats of formation of the larger species were evaluated by the G3 method from hydrogenation reactions (values at 298 K, in kcal/mol with estimated error bars of+/-3 kcal/mol): DeltaHf(B7H72-)=51.8; DeltaHf(B8H82-)=46.1; DeltaHf(B9H92-)=24.4; DeltaHf(B10H102-)=-12.5; DeltaHf(B11H112-)=-11.8; DeltaHf(B12H122-)=-86.3; DeltaHf(B11H14-)=-57.3; and DeltaHf(B10H14)=18.7. A linear correlation between atomization energies of the dianions and energies of the BH units was found. The heats of formation of the ammonium salts of the anions and dianions were predicted using lattice energies (UL) calculated from an empirical expression based on ionic volumes. The UL values (0 K) of the BnHn2- dianions range from 319 to 372 kcal/mol. The values of UL for the B3H8- and B11H14- anions are 113 and 135 kcal/mol, respectively. The calculated lattice energies and gas-phase heats of formation of the constituent ions were used to predict the heats of formation of the ammonium crystal salts [BnHmy-][NH4+]y. These results were used to evaluate the thermodynamics of the H2 release reactions from the ammonium hydro-borate salts.     

Correlations of ion structure with multiple fragmentation pathways arising from collision‐induced dissociations of selected α‐hydroxycarboxylic acid anions

Under conditions of collision‐induced dissociation (CID), anions of α‐hydroxycarboxylic acids usually fragment to yield the distinctive hydroxycarbonyl anion (m/z 45) and/or the complementary product anion formed by neutral loss of formic acid (46 u). Further support for the known two‐step mechanism, involving an ion‐neutral complex for the formation of the hydroxycarbonyl anion from the carboxyl group, is herein provided by tandem mass spectrometric results and density functional theory computations on the glycolate, lactate and 3‐phenyllactate ions. A fourth, structurally related α‐hydroxycarboxylate ion, obtained by deprotonation of mandelic acid, showed only loss of carbon dioxide upon CID. Density functional theory computations on the mandelate ion indicated that similar energy inputs were required for a direct, phenyl‐assisted decarboxylation and a postulated novel rearrangement to a carbonate ester, which yielded the benzyl oxide ion upon loss of CO₂. Rearrangement of the glycolate ion led to expulsion of carbon monoxide, whereas the 3‐phenyllactate ion showed the loss of water and formation of the benzyl anion and the benzyl radical as competing processes. The fragmentation pathways proposed for lactate and 3‐phenyllactate are supported by isotopic labeling. The relative computed energies of saddle points and product ions for all proposed fragmentation pathways are consistent with the energies supplied during CID experiments and the observed relative intensities of product ions. The diverse reaction pathways characterized for this set of four α‐hydroxycarboxylate ions demonstrate that it is crucial to understand the effects of structural variations when attempting to predict the gas‐phase reactivity and CID spectra of carboxylate ions. Copyright © 2013 John Wiley & Sons, Ltd.     

Enthalpies of formation of gas-phase N3, N3-, N5+, and N5- from Ab initio molecular orbital theory, stability predictions for N5(+)N3(-) and N5(+)N5(-), and experimental evidence for the instability of N5(+)N3(-)

Ab initio molecular orbital theory has been used to calculate accurate enthalpies of formation and adiabatic electron affinities or ionization potentials for N3, N3-, N5+, and N5- from total atomization energies. The calculated heats of formation of the gas-phase molecules/ions at 0 K are DeltaHf(N3(2Pi)) = 109.2, DeltaHf(N3-(1sigma+)) = 47.4, DeltaHf(N5-(1A1')) = 62.3, and DeltaHf(N5+(1A1)) = 353.3 kcal/mol with an estimated error bar of +/-1 kcal/mol. For comparison purposes, the error in the calculated bond energy for N2 is 0.72 kcal/mol. Born-Haber cycle calculations, using estimated lattice energies and the adiabatic ionization potentials of the anions and electron affinities of the cations, enable reliable stability predictions for the hypothetical N5(+)N3(-) and N5(+)N5(-) salts. The calculations show that neither salt can be stabilized and that both should decompose spontaneously into N3 radicals and N2. This conclusion was experimentally confirmed for the N5(+)N3(-) salt by low-temperature metathetical reactions between N5SbF6 and alkali metal azides in different solvents, resulting in violent reactions with spontaneous nitrogen evolution. It is emphasized that one needs to use adiabatic ionization potentials and electron affinities instead of vertical potentials and affinities for salt stability predictions when the formed radicals are not vibrationally stable. This is the case for the N5 radicals where the energy difference between vertical and adiabatic potentials amounts to about 100 kcal/mol per N5.     

Structural assignment of isomeric 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione mono- and disulfonic acids by liquid chromatography electrospray and atmospheric pressure chemical ionization tandem mass spectrometry

Positionally isomeric 2-(2-quinolinyl)-1H-indene-1,3(2H)-dione mono- and disulfonic acids give rise to similar electrospray ionization (ESI) and atmosphere pressure chemical ionization (APCI) mass spectra, which show very abundant MH(+) ions and negligible fragmentation. The MH(+) ions of these isomeric acids exhibit notably different behavior under collision-induced dissociation (CID) conditions. The acids with a sulfonic group at position 8' in the quinoline moiety, adjacent to the N-atom, exhibit highly abundant [MH - H(2)SO(3)](+) ions (m/z 272 for the mono- and m/z 352 for the disulfonic acids), which are of lower abundance in the CID spectra of isomers with the SO(3)H group at other positions, remote from the nitrogen atom. The latter isomers undergo efficient eliminations of SO(3) and HSO(3). The isomeric diacids with one SO(3)H group at position 4 of the indene-1,3(2H)-dione moiety, adjacent to one of the carbonyl groups, undergo highly efficient elimination of H(2)O. Mechanistic pathways, involving interactions between adjacent groups, are proposed for the above regiospecific fragmentations. Pronounced different behavior has been also observed in negative ion tandem mass spectrometric measurements of the sulfonic acids. The distinctive behavior of the isomeric acids was strongly pronounced when the measurements were performed with an ion trap mass spectrometer (LCQ), and much less so with a triple-stage quadrupole instrument (TSQ).     

A density-functional study of the mechanism for the diastereoselective epoxidation of chiral allylic alcohols by the titanium peroxy complexes

The epoxidation of three stereolabeled methyl-substituted chiral allylic alcohols with (1,2)A and/or (1,3)A allylic strain, namely 3-methylbut-3-en-2-ol (1a), pent-3-en-2-ol (1b), and 3-methylpent-3-en-2-ol (1c), have been studied by the density-functional theory method, B3LYP/6-31+G(d,p). For each substrate we calculated the two prereaction complexes with Ti(OH)(4)/MeOOH (the oxidant model for Ti(O-i-Pr)(4)/t-BuOOH), their threo and erythro transition states for oxygen transfer, and the corresponding product complexes. For substrate 1a, the erythro transition state is 0.91 kcal/mol of lower energy than the threo one; for substrates 1b and 1c, the threo compared to the erythro transition states are by 1.05 and 0.21 kcal/mol more favorable, respectively. The threo/erythro product ratios have been estimated from the computed free energies for the competing threo and erythro transition states 3a-c in CH(2)Cl(2) solution to be 12:88 (1a), 92:8 (1b), and 77:23 (1c), which are in good accordance with the experimental values 22:78 (1a), 91:9 (1b), and 83:17 (1c). The diastereoselectivity of this diastereoselective oxyfunctionalization is rationalized in terms of the competition between (1,3)A and (1,2)A strain and the electronic advantage for the spiro transition state. In addition, solvent effects are also play a role for the diastereoselectivity at the same time.     

Isomeric solid enols on ring- and amide-carbonyls of substituted 2-carbanilido-1,3-indandiones

Both isomeric enols on ring carbonyl (5b) and on amide carbonyl (6b) derived from N-p-methoxyphenyl-2-carbamido-1,3-indandione (4b) were isolated, and their X-ray structures were determined. X-ray diffraction of the N-o,p-dimethoxy analogue indicated a disorder ascribed to the presence of a 6:4 mixture of 5c and 6c. Calculation (B3LYP/6-31+G*) gave good agreement with observed geometries. The calculated energies indicated that enols 6 are more stable by <1 kcal/mol than enols 5 and much more stable than amides 4.     

Host-defence skin peptides of the Australian Common Froglet Crinia signifera: sequence determination using positive and negative ion electrospray mass spectra

The amino acid sequences of seven signiferin peptides are provided by consideration of tandem mass spectrometric (MS/MS) data for the respective MH+ and [M--H]- precursor ions. These methods do not differentiate between isomeric residues Leu and Ile; these were identified using the Edman degradation technique. The sequence of signiferin 1, a new smooth muscle contracting neuropeptide (RLCIPYIIPC-OH) containing a disulfide bridge, is best determined using the collision-induced dissociation (CID) spectrum of the [M--H]- anion. The initial fragmentation of this system involves loss of H2S2, which furnishes an open-chain system that is readily sequenced using the alpha and beta backbone cleavage anions.     

The effect of substitutents on the strain energies of small ring compounds

The effect of substitutents on the strain energy (SE) of cyclic molecules is examined at the CBS, G2, and G2(MP2) levels of theory. Alkyl substitutents have a meaningful effect upon the SE of small ring compounds. gem-Dimethyl substitution lowers the strain energy of cyclopropanes, cyclobutanes, epoxides, and dimethyldioxirane (DMDO) by 6-10 kcal/mol relative to an unbranched acyclic reference molecule. The choice of the reference compound is especially important for geminal electronegative substitutents. The SE of 1,1-difluorocyclopropane is estimated to be 20.5 kcal/mol relative to acyclic reference molecule 1,3-difluoropropane but is 40.7 kcal/mol with respect to the thermodynamically more stable (DeltaE = -20.2 kcal/mol) isomeric reference compound 2,2-difluoropropane. The SE of dioxirane (DO) is estimated to be approximately 18 kcal/mol while the SE of DMDO is predicted to be approximately equal to 11 kcal/mol by using homodesmotic reactions that maintain a balanced group equivalency. The total energy (CBS-APNO) of DMDO is 2.6 kcal/mol lower than that of isomeric 1,2-dioxacyclopentane that has an estimated SE of 5 kcal/mol. The thermodynamic stability of DMDO is a consequence of its relatively strong C-H (BDE = 102.7 kcal/mol) and C-CH(3) (BDE = 98.9 kcal/mol) bonds. By comparison, the calculated sec-C-H and -C-CH(3) G2 bond dissociation energies in propane are 100.3 and 90.5 kcal/mol.     

Determination of N-NO bond dissociation energies of N-methyl-N-nitrosobenzenesulfonamides in acetonitrile and application in the mechanism analyses on NO transfer

The heterolytic and homolytic N-NO bond dissociation energies of seven substituted N-methyl-N-nitrosobenzenesulfonamides (abbreviated as G-MNBS, G = p-OCH(3), p-CH(3), p-H, p-Cl, p-Br, 2,5-2Cl, m-NO(2)) in acetonitrile solution were evaluated for the first time by using titration calorimetry and relative thermodynamic cycles according to Hess' law. The results show that the energetic scales of the heterolytic and homolytic N-NO bond dissociation energies of G-MNBS in acetonitrile solution cover the ranges from 44.3 to 49.5 and from 33.0 to 34.9 kcal/mol for the neutral G-MNBS, respectively, which indicates that N-methyl-N-nitrosobenzenesulfonamides are much easier to release a NO radical (NO(*)) than to release a NO cation (NO(+)). The estimation of the heterolytic and homolytic (N-NO)(-)(*) bond dissociation energies of the seven G-MNBS radical anions in acetonitrile solution gives the energetic ranges of -15.8 to -12.9 and -3.1 to 1.8 kcal/mol for the (N-NO)(-)(*) bond homolysis and heterolysis, respectively, which means that G-MNBS radical anions are very unstable at room temperature and able to spontaneously or easily release a NO radical or NO anion (NO(-)), but releasing a NO radical is easier than releasing NO anion. These determined N-NO bond dissociation energies of G-MNBS and their radical anions have been successfully used in the mechanism analyses of NO transfer from G-MNBS to 3,6-dibromocarbazole and the reactions of NO with the substituted N-methyl-benzenesulfonamide nitranions (G-MBSN(-)) in acetonitrile solution.     

Conformational analysis of 1-butyl-3-methylimidazolium by CCSD(T) level ab initio calculations: effects of neighboring anions

Conformational energies for the butyl group of 1-butyl-3-methylimidazolium (bmim) were calculated by high-level ab initio methods. Estimated relative energies for the TT, GT and G'T rotamers of an isolated bmim cation at the CCSD(T)/cc-pVTZ level are 0.0 -0.02 and -0.50 kcal/mol, respectively. The close contact of a Cl anion to theC(2)-H of imidazolium considerably increases the relative stability of the GT rotamer. Estimated relative energies for the three rotamers of the [bmim]Cl complex, in which the Cl anion exists close to the C(2)-H, are 0.0, -1.61 and -0.25 kcal/mol, respectively. The GT rotamer is favored by the strong attractive electrostatic interaction between the bmim cation and Cl anion. The C(2)-H group in the GT rotamer has a larger positive charge compared with those in the TT and G'T rotamers. The contact of a Br anion to the C(2)-H also stabilizes the GT rotamer. The effects of the Cl anion close to the C(4)-Hand C(5)-Hare small. The anion effects suggest that the GT rotamer is the most stable in ionic liquids. The positive charge on imidazolium ring does not largely change the conformational energies. Estimated relative energies for the three rotamers of N-butylimidazole (0.0, -0.29 and -0.75 kcal/mol, respectively) are not largely different from those for isolated bmim. Calculated MP2/cc-pVTZ level torsional potential for the C im-N im-C-C bond has a minimum when the torsional angle is close to 90 degrees. Coplanar conformation is not a stable structure. Calculated torsional barrier height between the two nonplanar minima is less than 1 kcal/mol.     

Computer simulation of amino acid sorption on carbon nanotubes

A computer simulation of complexes of (6,6) open carbon nanotubes (CNTs) with neutral molecules, zwitterions and glycine, alanine, and phenylalanine amino acid anions is performed. In starting structures amino acids are arranged in three types: on the external side face, the open end, and inside CNT. The structure is optimized within the density functional theory with regard to the GD3 dispersion correction with and without taking into account solvation effects. It is found that the greatest CNT–amino acid interaction occurs in the neutral aqueous medium at dissociative chemisorption of the zwitterion (adsorption energy 80-90 kcal/mol) and in the basic medium at anion chemisorption (energy ~48-50 kcal/mol) on the open CNT end.     

Experimental and theoretical studies of potassium cation interactions with the acidic amino acids and their amide derivatives

The binding of K(+) to aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), and glutamine (Gln) is examined in detail by studying the collision-induced dissociation (CID) of the four potassium cation-bound amino acid complexes with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Formed by electrospray ionization, these complexes have energy-dependent CID cross sections that are analyzed to provide 0 K bond energies after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Quantum chemical calculations for a number of geometric conformations of each K(+)(L) complex are determined at the B3LYP/6-311+G(d,p) level with single-point energies calculated at B3LYP, B3P86, and MP2(full) levels using a 6-311+G(2d,2p) basis set. Theoretical bond dissociation energies are in good agreement with the experimental values. This coordinated examination of both experimental work and quantum chemical calculations allows for a comprehensive understanding of the molecular interactions of K(+) with the Asx and Glx amino acids. K(+) binding affinities for the amide complexes are systematically stronger than those for the acid complexes by 9+/-1 kJ/mol, which is attributed to an inductive effect of the OH group in the carboxylic acid side chain. Additionally, the K(+) binding affinity for the longer-chain amino acids (Glx) is enhanced by 5+/-1 kJ/mol compared to the shorter-chain Asx because steric effects are reduced. Further, a detailed comparison between experimental and theoretical results reveals interesting differences in the binding of K(+) and Na(+) to these amino acids.     

Formaldehyde oxime <--> nitrosomethane tautomerism

Formaldehyde oxime <--> nitrosomethane tautomerism, isomeric nitrone, and their common cations and anions are studied with Gaussian-2 theory using MP2(full)/6-31G geometries and with density functional theory using B3LYP/6-311+G**. Geometrical parameters, harmonic vibrational frequencies, relative stabilities, conformational stabilities, and ionization energies are compared with experimental gas-phase data when available. The formaldehyde oxime <--> nitrosomethane tautomerism is compared with the amide <--> imidol, imine <--> enamine, keto <--> enol, and nitro <--> aci-nitro tautomeric processes. Solvent effects are estimated by the self-consistent isodensity polarizable continuum model (SCIPCM). The influence of hydrogen bonding interactions with the solvent is addressed by including two water molecules. In the final evaluation, formaldehyde oxime is 15.8 kcal/mol more stable than nitrosomethane when the aqueous solvation correction of 3.8 kcal/mol is applied to the G2 energies. Unsolvated formaldehyde oxime is estimated to be 11.1 kcal/mol more stable than nitrone. The estimated gas-phase ionization energies (G2) are 362.5 kcal/mol for formaldehyde oxime, 350.6 kcal/mol for nitrosomethane, and 351.4 kcal/mol for nitrone.     

Accurately solving the electronic Schrodinger equation of small atoms and molecules using explicitly correlated (r12-)MR-CI. VIII. Valence excited states of methylene (CH2)

We compute the adiabatic transition energies of methylene (CH(2)) from the ground state to the lowest electronically excited valence states using the r(12)-MR-ACPF-2 method with a large basis set and an extended reference space. We recall that this method aims at reaching the basis-set and full configuration interaction (CI) limits simultaneously. Our best excitation energies, T(e) (T(0)), are 9.22 (8.87) (a (1)A(1), corrected for relativistic and adiabatic effects), 31.98 (31.86) (b (1)B(1)), and 57.62 (57.18) kcal mol(-1) (c (1)A(1)) (both uncorrected). We are able to reach the respective basis-set limits that closely that the remaining errors of our (uncorrected) calculations are clearly due to the MR-ACPF-2 method. While we are unable to assess the error of the latter method in a systematic way, we still believe that it is rather unlikely that the errors of our excitation energies exceed +/-0.10 kcal mol(-1). We finally observe that our (corrected) a state values deviate by only -0.10 (-0.10) kcal mol(-1) from the results of Csaszar et al. [J. Chem. Phys. 118, 10631 (2003)]--who did careful extrapolations to the valence full-CI and basis-set limits and added a correction for the core correlation--and that the deviation from experiment is only -0.13 (-0.13) kcal mol(-1). From these excellent agreements we conclude that our excitation energies to the b and c states are similarly accurate.     

Mass spectral study of alkali-cationized Boc-carbo-beta3-peptides by electrospray tandem mass spectrometry

Electrospray tandem mass spectrometry was used to study the dissociation reactions of [M+Cat]+ (Cat = Na+ and Li+) of Boc-carbo-beta3-peptides. The collision-induced dissociation (CID) spectra of [M+Cat-Boc]+ of these peptides are found to be significantly different from those of [M+H-Boc]+ ions. The spectra are more informative and display both C- and N-terminus metallated ions in addition to characteristic fragment ions of the carbohydrate moiety. Based on the fragmentations observed in the CID spectra of the [M+Cat-Boc]+ ions, it is suggested that the dissociation involves complexes in which the metal ion is coordinated in a multidentate arrangement involving the carbonyl oxygen atoms. The CID spectra of [M+Cat-Boc]+ ions of the peptide acids show an abundant N-terminal rearrangement ion [b(n)+17+Cat]+ which is absent for esters. Further, two pairs of positionally isomeric Boc-carbo-beta3-peptide acids, Boc-NH-Caa(S)-beta-hGly-OH (11) and Boc-NH-beta-hGly-Caa(S)-OH (12), and [Boc-NH-Caa(S)-beta-hGly-Caa(S)-beta-hGly-OH] (13) and [Boc-NH-beta-hGly-Caa(S)-beta-hGly-Caa(S)-OH] (14), were differentiated by the CID of [M+Cat-Boc]+ ions. The CID spectra of compounds 11 and 13 are significantly different from those of 12 and 14, respectively. The abundance of [b(n)+17+Cat]+ ions is higher for peptide acids 12 and 14 with a sugar group at the C-terminus when compared to 11 and 13 which contain a sugar moiety at the N-terminus. The observed differences between the CID spectra of these isomeric peptides are attributed to the difference in the preferential site of metal ion binding and also on the structure of the cyclic intermediate involved in the formation of the rearrangement ion.