Heats of formation of HCCl3, HCCl2Br, HCClBr2, HCBr3, and their fragment ions studied by threshold photoelectron photoion coincidence

The dissociative photoionization onsets for Cl and Br loss reactions were measured for HCCl3, HCCl2Br, HCClBr2, and HCBr3 by threshold photoelectron photoion coincidence (TPEPICO) in order to establish the heats of formation of the mixed halides as well as the following fragment ions: HCCl2(+), HCClBr(+), HCBr2(+). The first zero Kelvin onsets were measured with a precision of 10 meV. The second onsets, which are in competition with the lower energy onsets, were established with a precision of 60 meV. Because both the chloroform and bromoform have relatively well established heats of formation, these measurements provide a route for establishing the heats of formation of the mixed halomethanes within uncertainties of less than 5 kJ mol(-1).     

Thermochemistry and dissociative photoionization of Si(CH3)4, BrSi(CH3)3, ISi(CH3)3, and Si2(CH3)6 studied by threshold photoelectron-photoion coincidence spectroscopy

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to study the dissociation kinetics and thermochemistry of Me(4)Si, Me(6)Si(2), and Me(3)SiX, (X = Br, I) molecules. Accurate 0 K dissociative photoionization onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distribution, both of them analyzed and simulated in terms of the statistical RRKM theory and DFT calculations. The average enthalpy of formation of trimethylsilyl ion, Delta fH(o)298K(Me(3)Si(+)) = 617.3 +/- 2.3 kJ/mol, has been determined from the measured onsets for methyl loss (10.243 +/- 0.010 eV) from Me(4)Si, and Br and I loss from Me(3)SiBr (10.624 +/- 0.010 eV) and Me(3)SiI (9.773 +/- 0.015 eV), respectively. The methyl loss onsets for the trimethyl halo silanes lead to Delta fH(o)298K(Me(2)SiBr(+)) = 590.3 +/- 4.4 kJ/mol and Delta fH(o)298K(Me(5)Si(2)(+)) = 487.6 +/- 6.2 kJ/mol. The dissociative photoionization of Me(3)SiSiMe(3) proceeds by a very slow Si-Si bond breaking step, whose rate constants were measured as a function of the ion internal energy. Extrapolation of this rate constant to the dissociation limit leads to the 0 K dissociation onset (9.670 +/- 0.030 eV). This onset, along with the previously determined trimethylsilyl ion energy, leads to an enthalpy of formation of the trimethylsilyl radical, Delta fH(o)298K(Me(3)Si(*)) = 14.0 +/- 6.6 kJ/mol. In combination with other experimental values, we propose a more accurate average value for Delta fH(o)298K(Me(3)Si(*)) of 14.8 +/- 2.0 kJ/mol. Finally, the bond dissociation enthalpies (DeltaH(298K)) Si-H, Si-C, Si-X (X=Cl, Br, I) and Si-Si are derived and discussed in this study.     

Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.     

Threshold photoelectron-photoion coincidence spectroscopy: dissociation dynamics and thermochemistry of Ge(CH3)4, Ge(CH3)3Cl, and Ge(CH3)3Br

Threshold photoelectron-photoion coincidence spectroscopy (TPEPICO) has been used to investigate the gas-phase ionic dissociation energies and thermochemistry of Me4Ge and Me3GeX, (Me = methyl; X = Cl, Br) molecules. The 0 K dissociation onsets for these species have been measured from the breakdown diagram and the ion time-of-flight distributions, which were modeled with the statistical RRKM theory and DFT calculations. The measured 0 K dissociative photoionization onsets were as follows: Me3Ge+ + Me (9.826 +/- 0.010 eV); Me3Ge+ + Cl (10.796 +/- 0.040 eV); Me3Ge+ + Br (10.250 +/- 0.011 eV); Me2GeCl+ + Me (10.402 +/- 0.010 eV); and Me2GeBr+ + Me (10.333 +/- 0.020 eV). These onsets were used to obtain new values for delta(f)H(degrees)298 (in kJ/mol) of the neutral molecules Me3GeCl (-239.8 +/- 5.7) and Me3GeBr (-196.5 +/- 4.3), and also for the following ionic species: Me3Ge+ (682.3 +/- 4.1), Me2GeCl+ (621.1 +/- 5.8), and Me2GeBr+ (657.8 +/- 4.7).     

Energetics of C-Cl, C-Br, and C-I bonds in haloacetic acids: enthalpies of formation of XCH2COOH (X=CI, Br, I) compounds and the carboxymethyl radical

The standard molar enthalpies of formation of chloro-, bromo-, and iodoacetic acids in the crystalline state, at 298.15 K, were determined as deltafH(o)m(C2H3O2Cl, cr alpha)=-(509.74+/- 0.49) kJ x mol(-1), deltafH(o)m(C2H3O2Br, cr I)-(466.98 +/- 1.08) kJ x mol(-1), and deltafH(o)m (C2H3O2I, cr)=-(415.44 +/- 1.53) kJ x mol(-1), respectively, by rotating-bomb combustion calorimetry. Vapor pressure versus temperature measurements by the Knudsen effusion method led to deltasubH(o)m(C2H3O2Cl)=(82.19 +/- 0.92) kJ x mol(-1), deltasubH(o)m(C2H3O2Br)=(83.50 +/- 2.95) kJ x mol(-1), and deltasubH(o)m-(C2H3O2I) = (86.47 +/- 1.02) kJ x mol(-1), at 298.15 K. From the obtained deltafH(o)m(cr) and deltasubH(o)m values it was possible to derive deltafH(o)m(C2H3O2Cl, g)=-(427.55 +/- 1.04) kJ x mol(-1), deltafH(o)m (C2H3O2Br, g)=-(383.48 +/- 3.14) kJ x mol(-1), and deltafH(o)m(C2H3O2I, g)=-(328.97 +/- 1.84) kJ x mol(-1). These data, taken with a published value of the enthalpy of formation of acetic acid, and the enthalpy of formation of the carboxymethyl radical, deltafH(o)m(CH2COOH, g)=-(238 +/- 2) kJ x mol(-1), obtained from density functional theory calculations, led to DHo(H-CH2COOH)=(412.8 +/- 3.2) kJ x mol(-1), DHo(Cl-CH2COOH)=(310.9 +/- 2.2) kJ x mol(-1), DHo(Br-CH2COOH)=(257.4 +/- 3.7) kJ x mol(-1), and DHo(I-CH2COOH)=(197.8 +/- 2.7) kJ x mol(-1). A discussion of the C-X bonding energetics in XCH2COOH, CH3X, C2H5X, C2H3X, and C6H5X (X=H, Cl, Br, I) compounds is presented.     

Heats of formation of the propionyl ion and radical and 2,3-pentanedione by threshold photoelectron photoion coincidence spectroscopy

The dissociative photoionization onsets for the formation of the propionyl ion (C(2)H(5)CO(+)) and the acetyl ion (CH(3)CO(+)) were measured from energy selected butanone and 2,3-pentanedione ions using the technique of threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Ion time-of-flight (TOF) mass spectra recorded as a function of the ion internal energy permitted the construction of breakdown diagrams, which are the fractional abundances of ions as a function of the photon energy. The fitting of these diagrams with the statistical theory of unimolecular decay permitted the extraction of the 0 K dissociation limits of the first and second dissociation channels. This procedure was tested using the known energetics of the higher energy dissociation channel in butanone that produced the acetyl ion and the ethyl radical. By combining the measured dissociative photoionization onsets with the well-established heats of formation of CH(3)(*), CH(3)CO(+), CH(3)CO(*), and butanone, the 298 K heats of formation, Delta(f)H degrees (298K), of the propionyl ion and radical were determined to be 618.6 +/- 1.4 and -31.7 +/- 3.4 kJ/mol, respectively, and Delta(f)H degrees (298K)[2,3-pentanedione] was determined to be -343.7 +/- 2.5 kJ/mol. This is the first experimentally determined value for the heat of formation for 2,3-pentanedione. Ab initio calculations at the Weizmann-1 (W1) level of theory predict Delta(f)H degrees (298K) values for the propionyl ion and radical of 617.9 and -33.3 kJ/mol, respectively, in excellent agreement with the measured values.     

A photodissociation study of CH2BrCl in the A-band using the time-sliced ion velocity imaging method

Employing a high-resolution (velocity resolution deltanu/nu<1.5%) time-sliced ion velocity imaging apparatus, we have examined the photodissociation of CH2BrCl in the photon energy range of 448.6-618.5 kJ/mol (193.3-266.6 nm). Precise translational and angular distributions for the dominant Br(2P32) and Br(2P12) channels have been determined from the ion images observed for Br(2P32) and Br(2P12). In confirmation with the previous studies, the kinetic-energy distributions for the Br(2P12) channel are found to fit well with one Gaussian function, whereas the kinetic- energy distributions for the Br(2P32) channel exhibit bimodal structures and can be decomposed into a slow and a fast Gaussian component. The observed kinetic-energy distributions are consistent with the conclusion that the formation of the Br(2P32) and Br(2P12) channels takes place on a repulsive potential-energy surface, resulting in a significant fraction (0.40-0.47) of available energy to appear as translational energy for the photo fragments. On the basis of the detailed kinetic-energy distributions and anisotropy parameters obtained in the present study, together with the specific features and relative absorption cross sections of the excited 2A', 1A", 3A', 4A', and 2A" states estimated in previous studies, we have rationalized the dissociation pathways of CH2BrCl in the A-band, leading to the formation of the Br(2P32) and Br(2P12) channels. The analysis of the ion images observed at 235 nm for Cl(2P(32,12)) provides strong evidence that the formation of Cl mainly arises from the secondary photodissociation process CH2Cl + hnu --> CH2 + Cl.     

A computational study of the thermochemistry of bromine- and iodine-containing methanes and methyl radicals

Bromo- and iodomethanes and the corresponding halogenated methyl radicals have been investigated by ab initio methods. Geometries and vibrational frequencies were derived with quadratic configuration interaction methods at the QCISD/6-311G(d,p) level of theory, and energies via QCISD(T)/6-311+G(3df,2p). Core electrons were represented with relativistic effective potentials. Anharmonicity of the out-of-plane bending modes in the methyl radicals was taken into account by numerical integration of the Schr?dinger equation with potentials derived from relaxed scans of these modes. The results are in good accord with experimental data where available. Thermochemistry derived via isodesmic reactions referenced to CH3, CH4, and monohalomethanes yields excellent accord with new experiments on dihalomethanes and provides recommendations for the more poorly characterized tri- and tetrahalomethanes and halomethyl radicals. For the methanes CH2Br2, CHBr3, CBr4, CH2I2, CHI3, CI4, CH2BrI, CHBr2I, and CHBrI2 we compute DeltafH degrees (298) values of 4.3, 51.6, 110.6, 108.1, 208.5, 321.3, 56.8, 104.8, and 157.1 kJ mol(-1), respectively. For the methyl radicals CH2Br, CHBr2, CBr3, CH2I, CHI2, CI3, CHBrI, CBr2I, and CBrI2 we compute DeltafH degrees (298) values of 166.6, 191.7, 224.0, 217.2, 290.4, 369.1, 241.6, 320.8, and 272.3 kJ mol(-1), respectively. Recommended confidence limits are +/-3 kJ mol(-1) per Br or I atom. Trends in these values and the corresponding C-H bond strengths are discussed and compared with prior experiments, empirical estimation schemes, and ab initio calculations.     

PX4+, P2X5+, and P5X2+ (X=Br,I) salts of the superweak Al(OR)4- anion [R=C(CF3)3

PX(4) (+)[Al(OR)(4)](-) (X=I: 1 a, X=Br: 1 b) was prepared from X(2), PX(3), and Ag[Al(OR)(4)] [R=C(CF(3))(3)] in CH(2)Cl(2) at -30 degrees C in 69-86 % yield. P(2)X(5) (+) salts were prepared from 2 PX(3) and Ag[Al(OR)(4)] in CH(2)Cl(2) at -30 degrees C yielding almost quantitatively P(2)X(5) (+)[Al(OR)(4)](-) (X=I: 3 a, X=Br: 3 b). The phosphorus-rich P(5)X(2) (+) salts arose from the reaction of cold (-78 degrees C) mixtures of PX(3), P(4), and Ag[Al(OR)(4)] giving P(5)X(2) (+)[Al(OR)(4)](-) (X=I: 4 a, X=Br: 4 b) with a C(2v)-symmetric P(5) cage. Silver salt metathesis presumably generated unstable PX(2) (+) cations from PX(3) and Ag[Al(OR)(4)] (X=Br, I) that acted as electrophilic carbene analogues and inserted into the Xbond;X (Pbond;X/Pbond;P) bond of X(2) (PX(3)/P(4)) leading to the highly electrophilic and CH(2)Cl(2)-soluble PX(4) (+) (P(2)X(5) (+)/P(5)X(2) (+)) salts. Reactions that aimed to synthesize P(2)I(3) (+) from P(2)I(4) and Ag[Al(OR)(4)] instead led to anion decomposition and the formation of P(2)I(5)(CS(2))(+)[(RO)(3)Al-F-Al(OR)(3)](-) (5). All salts were characterized by variable-temperature solution NMR studies (3 b also by (31)P MAS NMR), Raman and/or IR spectroscopy as well as X-ray crystallography (with the exception of 4 a). The thermochemical volumes of the Pbond;X cations are 121 (PBr(4) (+)), 161 (PI(4) (+)), 194 (P(2)Br(5) (+)), 271 (P(2)I(5) (+)), and 180 A(3) (P(5)Br(2) (+)). The observed reactions were fully accounted for by thermochemical calculations based on (RI-)MP2/TZVPP ab initio results and COSMO solvation enthalpy calculations (CH(2)Cl(2) solution). The enthalpies of formation of the gaseous Pbond;X cations were derived as +764 (PI(4) (+)), +617 (PBr(4) (+)), +749 (P(2)I(5) (+)), +579 (P(2)Br(5) (+)), +762 (P(5)I(2) (+)), and +705 kJ mol(-1) (P(5)Br(2) (+)). The insertion of the intermediately prepared carbene analogue PX(2) (+) cations into the respective bonds were calculated, at the (RI-)MP2/TZVPP level, to be exergonic at 298 K in CH(2)Cl(2) by Delta(r)G(CH(2)Cl(2))=-133.5 (PI(4) (+)), -183.9 (PBr(4) (+)), -106.5 (P(2)I(5) (+)), -81.5 (P(2)Br(5) (+)), -113.2 (P(5)I(2) (+)), and -114.5 kJ mol(-1) (P(5)Br(2) (+)).     

Pressure-induced formation of molecular B2X4(μ-X)2 (X = Cl, Br, I) species

Boron(III) halides (BX(3), where X = F, Cl, Br, I) at ambient pressure conditions exist as strictly monomeric, trigonal-planar molecules. Using correlated ab initio calculations, the three heavier halides (X = Cl, Br, I) are shown to possess B(2)X(4)(μ-X)(2) local minima, isostructural with the diborane molecule. The calculated dissociation barrier of the B(2)I(4)(μ-I)(2) species [≈14 kJ/mol with CCSD(T)/cc-pVTZ] may be high enough to allow cryogenic isolation. The remaining dimer structures are more labile, with dissociation barriers of less than 6 kJ/mol. All three dimer species may be stabilized by application of external pressure. Periodic density functional theory calculations predict a new dimer-based P1 solid, which becomes more stable than the P6(3)/m monomer-derived solids at 5 (X = I) to 15 (X = Cl) GPa. Metadynamics simulations suggest that B(2)X(4)(μ-X)(2)-based solids are the kinetically preferred product of pressurization of the P6(3)/m solid.     

A photoelectron photoion coincidence study of the vinyl bromide and tribromoethane ion dissociation dynamics: heats of formation of C2H3+, C2H3Br, C2H3Br+, C2H3Br2+, and C2H3Br3

The threshold photoelectron photoion coincidence (TPEPICO) technique has been used to measure accurate dissociative photoionization onsets of vinyl bromide and 1,1,2-tribromoethane. The reactions investigated and their 0 K onsets are C2H3Br + hnu --> C2H3+ + Br (11.902 +/- 0.008 eV); C2H3Br3 + hnu --> C2H3Br2+ + Br (10.608 +/- 0.008 eV); and (C2H3Br3 + hnu --> C2H3Br+ + 2Br (12.301 +/- 0.035 eV). The vinyl ion heat of formation (Delta(f)H degrees 298K = 1116.1 +/- 3.0 kJ/mol) has been calculated using W1 theory and used as an anchor along with the measured dissociation energies to determine the heats of formation, Delta(f)H degrees 298K, in kJ/mol, of the following bromine-containing species: C2H3Br (74.1 +/- 3.1), C2H3Br+ (1021.9 +/- 3.1), C2H3Br2+ (967.1 +/- 4.0), and C2H3Br3 (53.5 +/- 4.3). These results represent accurate and consistent experimental determinations of heats of formation for these bromine-containing species, which serve to correct the discrepancies in the literature for C2H3Br and C2H3Br+ and provide the first experimental determination for the enthalpies of formation of C2H3Br2+ and C2H3Br3.     

TPEPICO spectroscopy of vinyl chloride and vinyl iodide: neutral and ionic heats of formation and bond energies

The 0 K dissociative ionization onsets of C2H3X --> C2H3(+) + X (X = Cl, I) are measured by threshold photoelectron-photoion coincidence spectroscopy. The heats of formation of C2H3Cl (Delta H(f,0K)(0) = 30.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 22.6 +/- 3.2 kJ mol(-1)) and C2H3I (Delta(H f,0K)(0) = 140.2 +/- 3.2 kJ mol(-1) and Delta(H f,298K)(0) = 131.2 +/- 3.2 kJ mol(-1)) and C- X bond dissociation enthalpies as well as those of their ions are determined. The data help resolve a longstanding discrepancy among experimental values of the vinyl chloride heat of formation, which now agrees with the latest theoretical determination. The reported vinyl iodide heat of formation is the first reliable experimental determination. Additionally, the adiabatic ionization energy of C2H3I (9.32 +/- 0.01 eV) is measured by threshold photoelectron spectroscopy.     

Dissociative photoionization mechanism of methanol isotopologues (CH3OH, CD3OH, CH3OD and CD3OD) by iPEPICO: energetics, statistical and non-statistical kinetics and isotope effects

The dissociative photoionization of energy selected methanol isotopologue (CH(3)OH, CD(3)OH, CH(3)OD and CD(3)OD) cations was investigated using imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. The first dissociation is an H/D-atom loss from the carbon, also confirmed by partial deuteration. Somewhat above 12 eV, a parallel H(2)-loss channel weakly asserts itself. At photon energies above 15 eV, in a consecutive hydrogen molecule loss to the first H-atom loss, the formation of CHO(+)/CDO(+) dominates as opposed to COH(+)/COD(+) formation. We see little evidence for H-atom scrambling in these processes. In the photon energy range corresponding to the B[combining tilde] and C[combining tilde] ion states, a hydroxyl radical loss appears yielding CH(3)(+)/CD(3)(+). Based on the branching ratios, statistical considerations and ab initio calculations, this process is confirmed to take place on the first electronically excited ?(2)A' ion state. Uncharacteristically, internal conversion is outcompeted by unimolecular dissociation due to the apparently weak Renner-Teller-like coupling between the X[combining tilde] and the ? ion states. The experimental 0 K appearance energies of the ions CH(2)OH(+), CD(2)OH(+), CH(2)OD(+) and CD(2)OD(+) are measured to be 11.646 ± 0.003 eV, 11.739 ± 0.003 eV, 11.642 ± 0.003 eV and 11.737 ± 0.003 eV, respectively. The E(0)(CH(2)OH(+)) = 11.6454 ± 0.0017 eV was obtained based on the independently measured isotopologue results and calculated zero point effects. The 0 K heat of formation of CH(2)OH(+), protonated formaldehyde, was determined to be 717.7 ± 0.7 kJ mol(-1). This yields a 0 K heat of formation of CH(2)OH of -11.1 ± 0.9 kJ mol(-1) and an experimental 298 K proton affinity of formaldehyde of 711.6 ± 0.8 kJ mol(-1). The reverse barrier to homonuclear H(2)-loss from CH(3)OH(+) is determined to be 36 kJ mol(-1), whereas for heteronuclear H(2)-loss from CH(2)OH(+) it is found to be 210 kJ mol(-1).     

Formation and internal energy distribution of the CH(A2Δ) radical by electron-impact dissociation of methyl halides

The dissociative excitation of CH₃X (X = Cl, Br, I) producing CH(A²Δ) is investigated by low-energy electron impact. The onsets of the CH(AX) emission from CH₃Cl, CH₃Br and CH₃I are 11.8 ± 0.3, 11.4 ± 0.3 and 11.2 ± 0.3 eV, respectively. It can be concluded that CH₃X → CH(A) + H₂(X) + X (X = Cl, Br, I) is the predominant process for formation of CH(A) near its onset. The internal energy distributions of CH(A) evaluated by means of a simulation analysis of the CH(AX) band are nearly independent of the impact energy for impinging electrons above 19 eV.     

Energetics of C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols. enthalpies of formation of XCH(2)CH(2)OH (X = F, Cl, Br, I) compounds and of the 2-hydroxyethyl radical

The energetics of the C-F, C-Cl, C-Br, and C-I bonds in 2-haloethanols was investigated by using a combination of experimental and theoretical methods. The standard molar enthalpies of formation of 2-chloro-, 2-bromo-, and 2-iodoethanol, at 298.15 K, were determined as Delta(f)H(degree)m(CH2CH2OH, l) = -315.5 +/- 0.7 kJ.mol-1, Delta(f)H(degree)mBrCH2CH2OH, l) = -275.8 +/- 0.6 kJ.mol-1, Delta(f)H(degree)m(ICH2CH2OH, l) = -207.3 +/- 0.7 kJ.mol-1, by rotating-bomb combustion calorimetry. The corresponding standard molar enthalpies of vaporization, Delta(vap)H(degree)m(ClCH2CH2OH) = 48.32 +/- 0.37 kJ.mol-1, Delta(vap)H(degree)m(BrCH2CH2OH) = 54.08 +/- 0.40 kJ.mol-1, and Delta(vap)H(degree)m(ICH2CH2OH) = 57.03 +/- 0.20 kJ.mol-1 were also obtained by Calvet-drop microcalorimetry. The condensed phase and vaporization enthalpy data lead to Delta(f)H(degree)m(ClCH2CH2OH, g) = -267.2 +/- 0.8 kJ.mol-1, Delta(f)H(degree)m(BrCH2CH2OH, g) = -221.7 +/- 0.7 kJ.mol-1, and Delta(f)H(degree)m(ICH2CH2OH, g) = -150.3 +/- 0.7 kJ.mol-1. These values, together with the enthalpy of selected isodesmic and isogyric gas-phase reactions predicted by density functional theory (B3LYP/cc-pVTZ) and CBS-QB3 calculations were used to derive the enthalpies of formation of gaseous 2-fluoroethanol, Delta(f)H(degree)m(FCH2CH2OH, g) = -423.6 +/- 5.0 kJ.mol-1, and of the 2-hydroxyethyl radical, Delta(f)H(degree)m(CH2CH2OH, g) = -28.7 +/- 8.0 kJ.mol-1. The obtained thermochemical data led to the following carbon-halogen bond dissociation enthalpies: DHo(X-CH2CH2OH) = 474.4 +/- 9.4 kJ.mol-1 (X = F), 359.9 +/- 8.0 kJ.mol-1 (X = Cl), 305.0 +/- 8.0 kJ.mol-1 (X = Br), 228.7 +/- 8.1 kJ.mol-1 (X = I). These values were compared with the corresponding C-X bond dissociation enthalpies in XCH2COOH, XCH3, XC2H5, XCH=CH2, and XC6H5. In view of this comparison the computational methods mentioned above were also used to obtain Delta(f)H(degree)m-594.0 +/- 5.0 kJ.mol-1 from which DHo(F-CH2COOH) = 435.4 +/- 5.4 kJ.mol-1. The order DHo(C-F) > DHo(C-Cl) > DHo(C-Br) > DHo(C-I) is observed for the haloalcohols and all other RX compounds. It is finally concluded that the major qualitative trends exhibited by the C-X bond dissociation enthalpies for the series of compounds studied in this work can be predicted by Pauling's electrostatic-covalent model.     

Dissociation dynamics and thermochemistry of tin species, (CH3)4Sn and (CH3)6Sn2, by threshold photoelectron-photoion coincidence spectroscopy

The dissociative photoionization of tetramethyltin (Me?Sn) and hexamethylditin (Me?Sn?) has been investigated by threshold photoelectron-photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me?Sn(+) dissociates rapidly by methyl loss, with a 0 K onset of E? = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me?Sn(?) loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me?Sn and Me?Sn? are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol?1: Δ(f)H(m)(o)(Me?Sn(+),g) = 746.3 ± 2.9; Δ(f)H(m)(o)(Me?Sn?(+),g) = 705.1 ± 7.5; Δ(f)H(m)(o)(Me?Sn(?),g) = 116.6 ± 9.7; Δ(f)H(m)(o)(Me?Sn,g) = 123.0 ± 16.5; Δ(f)H(m)(o)(MeSn(+),g) = 877.8 ± 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ·mol?1: BDE(Me?Sn-Me) = 284.1 ± 9.9; BDE(Me?Sn-SnMe?) = 252.6 ± 14.8.     

State-specific reactions of Cu(+)(1S, 3D) with CH3X and CF3X (X = Cl, Br, I): exploring the influence of dipole orientation on association and C-X bond activation

The reactions of gas-phase Cu(+)((1)S) and Cu(+)((3)D) with CF(3)X and CH(3)X (X = Cl, Br, and I) have been examined experimentally using the drift cell technique at 3.5 Torr in He at room temperature. State-specific product channels and overall bimolecular rate constants for depletion of the two Cu(+) states were determined using electronic state chromatography. The results showed that Cu(+)((1)S) participates exclusively in association with all of these neutrals, whereas, depending on the neutral, Cu(+)((3)D) initiates up to three bimolecular processes, resulting in the formation of CuX(+), CuC(H/F)(3)(+), and C(H/F)(3)X(+). Possible structures for the singlet association products were explored using density functional methods. These calculations indicated that Cu(+) preferentially associates with the labile halogen (Cl, Br, I) with all neutrals except CF(3)Cl, for which a "backside" geometry occurs in which Cu(+)((1)S) is weakly bound to the -CF(3) end of the molecule. All products observed on the triplet reaction surface can be understood in terms of either known or calculated thermochemical requirements. Product distributions and overall reaction efficiencies for C-X bond activation (X = Br, I) through Cu(+)((3)D) suggest that the orientation of the neutral dipole has little or no effect in controlling access to specific product channels. Likewise, second-order rate constants for reactions with X = Br and I indicate efficient depletion of Cu(+)((3)D) and do not exhibit the dramatic variations in reaction efficiency previously observed with CH(3)Cl and CF(3)Cl. These results suggest that C-X bond activation proceeds through a bond-insertion mechanism as opposed to direct abstraction.     

Density functional theory investigation of the reactions of isodihalomethanes (CH(2)X-X where X = Cl, Br, or I) with ethylene: substituent effects on the carbenoid behavior of the CH(2)X-X species

We investigated the chemical reactions of isodihalomethane (CH(2)X-X) and CH(2)X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH(2)X-X using density functional theory calculations. The CH(2)X-X species readily reacts with ethylene to give the cyclopropane product and an X(2) product via a one-step reaction with barrier heights of approximately 2.9 kcal/mol for CH(2)I-I, 6.8 kcal/mol for CH(2)Br-Br, and 8.9 kcal/mol for CH(2)Cl-Cl. The CH(2)X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH(2)X species. The isomerization reactions of CH(2)X-X had barrier heights of approximately 14.4 kcal/mol for CH(2)I-I, 11.8 kcal/mol for CH(2)Br-Br, and 9.1 kcal/mol for CH(2)Cl-Cl. We compare our results for the CH(2)X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH(2)ZnX) and Li-type carbenoids (LiCH(2)X) and discuss their differences and similarities as methylene transfer agents.     

2,3-二氮杂二环-[2,2,2]-辛-2-烯与卤素分子间电荷转移相互作用的量子化学研究

对2,3-二氮杂二环-[2,2,2]-辛-2-烯 (DBO) 与Cl2、Br2和I2 3种卤素分子形成的s型电荷转移复合物进行了不同方法的理论计算。结果表明, MP2方法结合LanL2DZ*基组可以很好地描述DBO与卤素分子间的相互作用。其中, DBO与I2的结合能为33.6 kJ/mol, 与DBO…Br2的结合能接近, 明显大于DBO…Cl2相应的值。几何结构变化以及布居分析均与这一结果吻合。针对DBO的结构, 对DBO与I2间 形成p型电荷转移复合物以及DBO…(I2)2的可能性进行了理论分析和预测。     

Water-catalyzed O-H insertion/HI elimination reactions of isodihalomethanes (CH2X-I, where X = Cl, Br, I) with water and the dehalogenation of dihalomethanes in water-solvated environments

A combined experimental and theoretical investigation of the ultraviolet photolysis of CH2XI (where X = Cl, Br, I) dihalomethanes in water is presented. Ultraviolet photolysis of low concentrations of CH2XI (where X = Cl, Br, I) in water appears to lead to almost complete conversion into CH2(OH)2 and HX and HI products. Picosecond time-resolved resonance Raman (ps-TR3) spectroscopy experiments revealed that noticeable amounts of CH2X-I isodihalomethane intermediates were formed within several picoseconds after photolysis of the CH2XI parent compound in mixed aqueous solutions. The ps-TR3 experiments in mixed aqueous solutions revealed that the decay of the CH2X-I isodihalomethane intermediates become significantly shorter as the water concentration increases, indicating that the CH2X-I intermediates may be reacting with water. Ab initio calculations found that the CH2X-I intermediates are able to react relatively easily with water via a water-catalyzed O-H insertion/HI elimination reaction to produce CH2X(OH) and HI products, with the barrier for these reactions increasing as X changes from Cl to Br to I. The ab initio calculations also found that the CH2X(OH) product can undergo a water-catalyzed HX elimination reaction to make H2C=O and HX products, with the barrier to reaction decreasing as X changes from Cl to Br to I. The preceding two water-catalyzed reactions produce the HI and HX leaving groups observed experimentally, and the H2C=O product further reacts with water to make the other CH2(OH)2 product observed in the photochemistry experiments. This suggests that that the CH2X-I intermediates react with water to form the CH2(OH)2 and HI and HX products observed in the photochemistry experiments. Ultraviolet photolysis of CH2XI (where X = Cl, Br, I) at low concentrations in water-solvated environments appears to lead to efficient dehalogenation and release of two strong acid leaving groups. We very briefly discuss the potential influence of this photochemistry in water on the decomposition of polyhalomethanes and halomethanols in aqueous environments.