Resonant two-photon ionization and ab initio conformational analysis of haloethyl benzenes (PhCH(2)CH(2)X,X=Cl,F)

The S(1) <-- S(0) transitions of the gaseous (2-fluoroethyl)-benzene (FEB) and (2-chloroethyl)-benzene (CEB) have been investigated using a combination of two-color resonant two-photon ionization and UV-UV hole burning spectroscopy. Both anti and gauche conformers have been identified on the basis of rotational band contour analysis supported by ab initio calculations on the ground and electronically excited states. The gauche origin band of FEB at 37,673 cm(-1) is redshifted 50 cm(-1) relative to the corresponding anti origin, while CEB origin bands overlap at 37,646 cm(-1). Relative conformational stability and populations in the jet have been estimated for both molecules, based on the intensity ratio of S(1) <-- S(0) band origin transitions. These are compared with a range of related molecules with the structural motif PhCH(2)CH(2)X (X=CH(3),CH(2)CH(3),NH(2),OH,COOH,CCH,CN). Theory and experimental results for FEB and CEB show repulsive interactions between the halogen substituents and the pi cloud of the phenyl rings destabilizing the gauche conformers, but the preference for the anti conformers is relatively modest. The gauche conformer origins show very different hybrid character: FEB is largely b type, while CEB is an ac hybrid in keeping with theoretically computed TM "rotations" (theta(elec)) of -7 degrees and -56 degrees , respectively. This difference is attributed largely to rotation of the side chain in opposite directions about the C(1)C(alpha) bond. Spectra of FEB(H(2)O) and CEB(H(2)O) single water clusters show evidence of an anti conformation in the host molecule.     

Reaction of dimethylplatinum(II) complexes with PhCH2CH2Br: Comparative reactivity with CH3CH2Br and PhCH2Br and synthesis of Pt(IV) complexes

Oxidative addition of 2‐phenylethylbromide (PhCH₂CH₂Br) to dimethylplatinum(II) complexes [PtMe₂(NN)] ( 1a , NN = 2,2′‐bipyridine (bpy); 1b , NN = 1,10‐phenanthroline (phen)) afforded the new organoplatinum(IV) complexes [PtMe₂(Br)(PhCH₂CH₂)(bpy)], as a mixture of trans ( 2a ) and cis ( 3a ) isomers, and [PtMe₂(Br)(PhCH₂CH₂)(phen)], as a mixture of trans ( 2b ) and cis ( 3b ) isomers, respectively. The new Pt(IV) complexes were readily characterized using multinuclear (¹H and ¹³C) NMR spectroscopy and elemental microanalysis. The crystal structure of 2a was further determined using X‐ray crystallography indicating an octahedral geometry around the platinum centre. A comparison of reactivity of RCH₂Br reagents (R = CH₃, Ph or PhCH₂) in their oxidative addition reactions with complex 1a , with an emphasis on the effects of the R groups of alkyl halides, was also conducted using density functional theory.     

Theoretical study of PhCH2O4CH2Ph: intermediate in the PhCH2O2 self-reaction

As a possible important intermediate during the oxidation of toluene in gas phase, the PhCH₂O₄CH₂Ph molecule has been invested on the structural parameters at CAM-B3LYP/6-311+g(df,pd) level, the vibrational frequencies at B3LYP-D3(BJ)/6-311+g(df,pd) level, and the potential energy surface of isomerization between main isomers at CCSD(T)-F12/cc-pVDZ-F12 level of theory. Ten PhCH₂O₄CH₂Ph isomers were located, in which the structural parameters of C-O₄-C skeleton are close to those in HO₄H and C₂H₅O₄C₂H₅. However, due to larger steric hindrance of aromatic groups, PhCH₂O₄CH₂Ph has a more open geometry with larger bond angles and dihedral angles in individual isomers. Meanwhile, it seems that the hyperconjugation appears to be stronger between the O-O bonds and aromatic groups, since the O-O bond lengths are slightly shorter than those found in HO₄H and C₂H₅O₄C₂H₅. Harmonic vibrational frequencies of PhCH₂O₄CH₂Ph isomers were first reported here and we found that the frequencies of O₄ torsion and O-O/C-O stretches in PhCH₂O₄CH₂Ph are consistent with those of CH₃O₄CH₃ and C₂H₅O₄C₂H₅. In the progress of isomerization, three transition states were located and a thermodynamic equilibrium between three main isomers may achieve at ambient temperature concerning the characters of energy barrier.

     

Comparison between CH3(CH2)15SH and CF3(CF2)3(CH2)11SH monolayers on electrodeposited silver

In this paper, two monolayers self-assembled on a silver substrate are compared: a monolayer of n-hexadecanethiol and a monolayer of n-11-perfluorobutylundecanethiol. The protecting properties of both monolayers have been extensively studied by X-ray photoelectron spectroscopy, contact angle, polarization modulation infrared reflection absorption spectroscopy, conventional electrochemical techniques (polarization curves and electrochemical impedance spectroscopy), and scanning vibrating electrode technique. Both monolayers were successfully self-assembled but organization is slightly different, the fluorinated segment introduces small disorganization. Nevertheless, good homogeneous corrosion protection is observed for each monolayer.     

Reaction mechanism of HSH and CH3SH with NH2CH2COCH2X (X = F and Cl) molecules

The reaction mechanism of model compounds H₂S and CH₃SH for cysteine proteases with NH₂CH₂COCH₂X (X = F and Cl) molecules has been investigated using DFT methods with B3LYP and B3PW91 hybrid density functionals at 6‐31+G* basis sets. The single point energy has been calculated for the above reactions with B3LYP and B3PW91 functionals using aug‐cc‐PVDZ infinite basis set in both gas and solution phases. The intrinsic reaction coordinates calculations have been performed to confirm that each transition state is linked by the desired reactants and products. The geometries and relative energies for various stationary points have been determined and discussed. The zero point vibrational energy corrections have been made to predict the reliable energy. The negative value of reaction energy indicates that the overall reaction profile is found to be exothermic. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Microwave spectrum, conformation and intramolecular hydrogen bonding of 2,2,2-trifluoroethanethiol (CF3CH2SH)

The microwave spectrum of 2,2,2-trifluoroethanethiol, CF3CH2SH, and of one deuterated species, CF3CH2SD, has been investigated in the 7-80 GHz spectral interval. The microwave spectra of the ground and three vibrationally excited states belonging to three different normal modes of one conformer were assigned for the parent species, and the vibrational frequencies of these fundamentals were determined by relative intensity measurements. Only the ground vibrational state was assigned for the deuterated species. The identified form has a synclinal arrangement for the H-S-C-C chain of atoms and the corresponding dihedral angle is 68(5) degrees from synperiplanar (0 degrees). A weak intramolecular hydrogen bond formed between the thiol (SH) group and one of the fluorine atoms is stabilizing this conformer. There is no evidence in the microwave spectrum for the H-S-C-C antiperiplanar form. The hydrogen atom of the thiol group should have the ability to tunnel between two equivalent synclinal potential wells, but no splittings of spectral lines due to tunneling were observed. The microwave work was augmented by quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory.     

The structures and stability of pentacoordinate germylenoid PhCH2(NH2)CH3GeLiF

The structures and stability of pentacoordinate germylenoid PhCH₂(NH₂)CH₃GeLiF were first theoretically studied by density functional theory. Two equilibrium structures, the three-membered ring and the p-complex structures, were located. The three-membered ring structure is more stable both in vacuum and in solvents (ether, THF, and acetone). The Ge–N coordination energies at the B3LYP/6-311+G(d,p) level are 35.8 and 7.9 kJ/mol in the three-membered ring and the p-complex structures, respectively. The insertion reactions with CH₃F indicate that germylenoid PhCH₂(NH₂)CH₃GeLiF is more stable than germylene PhCH₂(NH₂)CH₃Ge. The insertion barrier of PhCH₂(NH₂)CH₃GeLiF with CH₃F is higher than that of PhCH₃CH₃GeLiF, indicating that the amine coordination make PhCH₂(NH₂)CH₃GeLiF more stable.     

Asymmetric PhCH(CH3)CH2OOOOC(CH3)2Ph Tetroxide as an Intermediate in Chain-Termination Processes and Chemiluminescence Excitation in the Oxidation of Cumene

The process of chain termination in the oxidation of cumene was studied. With the use of the semiempirical PM3 method, the structures of primary and tertiary peroxide radicals (PhCH(CH₃)CH₂OO^· and Ph(CH₃)₂COO^·), the PhCH(CH₃)CH₂OOOOC(CH₃)₂Ph tetroxide (TO) (the product of the combination of the above radicals), and TO decomposition products were studied and their heats of formation (H ⁰ _f) were determined; the activation energy of TO decomposition was evaluated. Similar values of H ⁰ _f were obtained by the thermochemical method of group additivity. The PM3 calculation demonstrated that the irreversible decomposition of the asymmetric TO in a six-membered transition complex into the PhCH(CH₃)CHO aldehyde, the Ph(CH₃)COH alcohol, and O₂ is a synchronous process: dramatic changes in the bond lengths and bond orders occurred simultaneously. In this case, 100 kcal/mol was released, which is sufficient for the chemiexcitation of triplet R_–H=O and singlet O₂. A conclusion was drawn that a small impurity of PhCH(CH₃)CH₂OO^· primary radicals plays an important role in chain termination and is the only reason for the excitation of chemiluminescence.     

Reactions of CH3SH and (CH3)2S2 on the (0001) and (000) Surfaces of ZnO

Temperature-programmed desorption (TPD) was used to study the adsorption and reaction of CH3SH and (CH3)2S2 on the (0001) and (000) surfaces of ZnO. The interaction of these molecules with ZnO was found to be structure-sensitive. Both CH3SH and (CH3)2S2 adsorb dissociatively on ZnO(0001), forming adsorbed methylthiolate intermediates and only molecularly on ZnO(000). In the case of CH3SH, this result is consistent with that reported previously for the interaction of Br?nsted acids with ZnO and indicates that exposed cation-anion pairs are the active sites for dissociative adsorption. Only exposed Zn cations are required for the dissociative adsorption of (CH3)2S2. Methylthiolate species produced by dissociative adsorption of CH3SH and (CH3)2S2 on ZnO(0001) were found to undergo a variety of reaction pathways, including coupling to produce dimethyl sulfide and oxidation to formaldehyde, CO, and CO2. Pathways for the production of these various products are proposed.     

A study of Src SH2 domain protein-phosphopeptide binding interactions by electrospray ionization mass spectrometry

The noncovalent binding of various peptide ligands to pp60^src (Src) SH2 (Src homology 2) domain protein (12.9 ku) has been used as a model system for development of electrospray ionization mass spectrometry (ESI-MS) as a tool to study noncovalently bound complexes. SH2 motifs in proteins are critical in the signal transduction pathways of the tyrosine kinase growth factor receptors and recognize phosphotyrosine-containing proteins and peptides. ESI-MS with a magnetic sector instrument and array detection has been used to detect the protein-peptide complex with low-picomole sensitivity. The relative abundances of the multiply charged ions for the complex formed between Src SH2 protein and several nonphosphorylated and phosphorylated peptides have been compared. The mass spectrometry data correlate well to the measured binding constants derived from solution-based methods, indicating that the mass spectrometry-based method can be used to assess the affinity of such interactions. Solution-phase equilibrium constants may be determined by measuring the amount of bound and unbound species as a function of concentration for construction of a Scatchard graph. ESI-MS of a solution containing Src SH2 with a mixture of phosphopeptides showed the expected protein-phosphopeptide complex as the dominant species in the mass spectrum, demonstrating the method’s potential for screening mixtures from peptide libraries.     

Imaging the photodissociation of CH3SH in the first and second absorption bands: the CH3(X2A1)+SH(X2Pi) channel

The CH3(X2A1)+SH(X2Pi) channel of the photodissociation of CH3SH has been investigated at several wavelengths in the first 1 1A"<--X 1A' and second 2 1A"<--X1A' absorption bands by means of velocity map imaging of the CH3 fragment. A fast highly anisotropic (beta=-1+/-0.1) CH3(X2A1) signal has been observed in the images at all the photolysis wavelengths studied, which is consistent with a direct dissociation process from an electronically excited state by cleavage of the C-S bond in the parent molecule. From the analysis of the CH3 images, vibrational populations of the SH(X2Pi) counterfragment have been extracted. In the second absorption band, the SH fragment is formed with an inverted vibrational distribution as a consequence of the forces acting in the crossing from the bound 2 1A" second excited state to the unbound 1 1A" first excited state. The internal energy of the SH radical increases as the photolysis wavelength decreases. In the case of photodissociation via the first excited state, the direct production of CH3 leaves the SH counterfragment with little internal excitation. Moreover, at the longer photolysis wavelengths corresponding to excitation to the 1 1A" state, a slower anisotropic CH3 channel has been observed (beta=-0.8+/-0.1) consistent with a two step photodissociation process, where the first step corresponds to the production of CH3S(X2E) radicals via cleavage of the S-H bond in CH3SH, followed by photodissociation of the nascent CH3S radicals yielding CH3(X2A1)+S(X3P0,1,2).     

二苄基N,N-二甲基荒酸锡的合成及晶体结构研究

用二苄基二氯化锡与N,N-二甲基荒酸钠反应合成了化合物(PhCH2)2Sn[S2CN(CH3)2]2,通过X-射线确定了其晶体结构,晶胞参数为单斜晶系,空间群P21/n,a=13.926(5), b=9.832(4), c=17.080(7),β=103.541(6)°, V=2273.7(15)3, Z=4.     

High level ab initio and density functional theory study of bond selective dissociation of CH3SH and CH3CH2SH radical cations

Selective bond dissociation energies for CH₃SH and CH₃CH₂SH radical cations were evaluated with G1, G2, G2MP2, B3LYP, BLYP, and SVWN computational methods. It was determined that both G2 and CBSQ evaluate very accurate bond dissociation energies for thiol radical cations, while gradient-corrected BLYP computes the best energies of three employed DFT methods. For the CH₃CH₂SH radical cation, new, higher than previously estimated selective bond dissociation energies were suggested. Received: 10 September 1997 / Accepted: 9 September 1998 / Published online: 11 November 1998     

Formation of singly bonded PhCH2C60-C60CH2Ph dimers from 1,2-(PhCH2)HC60 via electroreductive C60-H activation

Singly bonded PhCH(2)C(60)-C(60)CH(2)Ph dimers are generated via controlled-potential bulk electroreduction and electrooxidation of 1,2-(PhCH(2))HC(60). The reaction mixture was purified by HPLC, and the isolated fraction was characterized with single-crystal X-ray diffractions, (1)H and (13)NMR, MS, elemental analysis, and cyclic voltammetry. It was shown that the fraction consists of two HPLC-inseparable PhCH(2)C(60)-C(60)CH(2)Ph regioisomers, which are assigned as the meso and racemic regioisomers. The bulk electrolysis processes for the formation of the dimers were followed by in situ cyclic voltammetry and were further corroborated with an in situ voltammetric titration of 1,2-(PhCH(2))HC(60) with tetra-n-butylammonium hydroxide (TBAOH), on the basis of which a reaction mechanism is proposed.     

单分子水对CH_2SH+NO_2反应机理影响的理论研究

在B3LYP/6-311++G(2df,p)水平下对单分子水参与下的CH_2SH+NO_2反应的微观机理进行了研究.为了获得更准确的能量信息,采用HL复合方法和CCSD(T)/aug-ccpvtz方法进行单点能校正.结果表明,加入单分子水后的CH_2SH+NO_2反应体系,共经过10条不同的反应路径,得到6种反应产物.与裸反应(CH_2SH+NO_2)相比,水分子在反应中起到了明显的正催化作用.不仅使生成产物trans-HONO的能垒(-52.84kJ·mol~(-1))降低了176.94kJ·mol~(-1),而且不需经过复杂的重排和异构化过程便可得到产物cis-HONO.在生成产物cis-HONO通道(Path3和Path4)中,活化能垒分别为143.65和126.70kJ·mol~(-1),而其裸反应的活化能垒却高达238.34kJ·mol~(-1).生成HNO_2的通道中(Path5和Path6)活化能垒分别为295.23和-42.19kJ·mol~(-1).其中Path6的无势垒过程使HNO_2也成为该反应的主要产物.另外,单分子水还可通过氢迁移的方式直接参与CH_2SH+NO_2的反应,活化能垒(TS7-TS10)分别为-10.62,151.03,186.22和155.10kJ·mol~(-1).除直接抽氢通道中的(Path8-Path10)外,其余反应通道均为放热反应,在热力学上是可行的.     

Dependence of glycine CH2 stretching frequencies on conformation, ionization state, and hydrogen bonding

We experimentally and theoretically examined the conformation, pH, and temperature dependence of the CH2 stretching frequencies of glycine (gly) in solution and in the crystalline state. To separate the effects of the amine and carboxyl groups on the CH2 stretching frequencies we examined the Raman spectra of 2,2,2-d3-ethylamine (CD3-CH2-NH2) and 3,3,3-d3-propionic acid (CD3-CH2-COOH) in D2O. The symmetric (nusCH2) and asymmetric (nuasCH2) stretching frequencies show a significant dependence on gly conformation. We quantified the relation between the frequency splitting (Delta = nuasCH2-nusCH2) and the xi angle which determines the gly conformational geometry. This relation allows us to determine the conformation of gly directly from the Raman spectral frequencies. We observe a large dependence of the nusCH2 and nuasCH2 frequencies on the ionization state of the amine group, which we demonstrate theoretically results from a negative hyperconjugation between the nitrogen lone pair and the C-H antibonding orbitals. The magnitude of this effect is maximized for C-H bonds trans to the nitrogen lone pair. In contrast, a small dependence of the CH2 stretching frequencies on the carboxyl group ionization state arises from delocalization of electron density from carboxyl oxygen to C-H bonding orbitals. According to our experimental observations and theoretical calculations the temperature dependence of the nusCH2 and nuasCH2 of gly is due to the change in the hydrogen-bonding strength of the amine/carboxyl groups to water.