On the weak O-H···halogen hydrogen bond: a rotational study of CH3CHClF···H2O

We measured the molecular beam Fourier transform microwave spectra of six isotopologues of the 1?:?1 adduct of CH(3)CHClF with water. Water prefers to form an O-H···F rather than an O-H···Cl hydrogen bond. This is just the contrary of what was observed in the chlorofluoromethane-water adduct, where an O-H···Cl link was formed (W. Caminati, S. Melandri, A. Maris and P. Ottaviani, Angew. Chem., Int. Ed., 2006, 45, 2438). The water molecule is linked with an O-H···F bridge to the fluorine atom, with r(F···H(w)) = 2.14 ?, and with two C-H···O contacts to the alkyl hydrogens with r(C(1)-H(1)···O(w)) = 2.75 ? and r(C(2)-H(2)···O(w)) = 2.84 ?, respectively. Besides the rotational constants, the quadrupole coupling constants of the chlorine atom have been determined. In addition, information on the internal dynamics has been obtained.     

Internal rotation and halogen bonds in CF3I···NH3 and CF3I···N(CH3)3 probed by broadband rotational spectroscopy

The rotational spectra of CF(3)I···NH(3) and CF(3)I···N(CH(3))(3) are measured between 6.7 and 18 GHz using a chirped-pulse Fourier transform microwave spectrometer. Transitions in each spectrum are assigned to A and E species associated with ground and excited internal rotor states respectively. Rotational constants, B(0), centrifugal distortion constants, D(J), D(Jm), D(JKm), nuclear quadrupole coupling constants of the (14)N and (127)I atoms, χ(aa)(N) and χ(aa)(I), are determined for each complex. D(JK) is additionally determined for CF(3)I···NH(3). Results are presented for both (14)N and (15)N-substituted isotopologues. All data are consistent with C(3v) symmetric top structures for both complexes. The nuclear quadrupole coupling constants of iodine are determined to be -2230.030(83) MHz and -2241.61(17) MHz in CF(3)I···(14)NH(3) and CF(3)I···(14)N(CH(3))(3) respectively. The data are interpreted through a model that accounts for the internal dynamics of the complexes in order to determine the length of the halogen bond between the iodine and nitrogen atoms, r(N···I). Values of r(N···I) are thus determined to lie in the ranges 3.054 ? > r(N···I) > 3.034 ? and 2.790 ? > r(N···I) > 2.769 ? for CF(3)I···NH(3) and CF(3)I···N(CH(3))(3) respectively.     

The halogen bond made visible: experimental charge density of a very short intermolecular Cl···Cl donor-acceptor contact

[ZnCl(2)(3,4,5-trichloropyridine)(2)] features short intermolecular Cl···Cl contacts between halogen atoms of different nature, and a charge density study provides experimental evidence for the accepted model of the halogen bonds: an arene-bonded Cl atom acts as a donor of electron density towards the "sigma hole" of a chlorido ligand attached to a neighbouring Zn(II) cation.     

Clˉ as the halogen bond acceptor: studies on strong halogen bonds

The Clˉ anion as the halogen bond acceptor, the diiodotetrafluoroethane I(CF₂)₂I and its derivatives I(MF₂)_nI (M = C, Si, Ge, Sn) as the halogen bond donor, and the strong halogen bonds could be formed. The halogen bonds between I(MF₂)_nI and Clˉ have been designed and investigated by Moller–Plesset perturbation/aug-cc-pVDZ calculations together with the aug-cc-pVDZ-pp basis set for iodine and stannum. The halogen bonds in the I(MF₂)_nI???Clˉ complexes are strong, which are apparently related to the group IV elements, becoming stronger along the sequence of M = Si, C, Ge, Sn. Accompanied with increasing number (n) of MF₂ unit, the halogen bonds (M = Si, Ge, Sn) also become stronger. The energy decomposition analyses reveal that the exchange energy contributes most in forming these halogen-bonded interactions. In the meantime, the electrostatic energy is also a significant factor for the I???Clˉ interactions. The halogen bonds of I(MF₂)nI???Clˉ(M = C, Ge, Sn) belong to partial-covalent interactions, while they are noncovalent interactions when M = Si.     

Infrared and microwave spectra of the acetylene-ammonia and carbonyl sulfide-ammonia complexes: a comparative study of a weak C-H···N hydrogen bond and an S···N bond

We report a combined high resolution infrared and microwave spectroscopic investigation of the acetylene-ammonia and carbonyl sulfide-ammonia complexes using a pulsed slit-nozzle multipass absorption spectrometer based on a quantum cascade laser and a pulsed nozzle beam Fourier transform microwave spectrometer, respectively. The ro-vibrational transitions of the acetylene-ammonia complex have been measured at 6 μm in the vicinity of the ν(4) band of ammonia for the first time. The previously reported pure rotational transitions have been extended to higher J and K values with (14)N nuclear quadrupole hyperfine components detected and analyzed. The spectral analysis reveals that acetylene binds to ammonia through a C-H···N weak hydrogen bond to form a C(3v) symmetric top, consistent with the previous microwave [Fraser et al., J. Chem. Phys., 1984, 80, 1423] and infrared spectroscopic study at 3 μm [Hilpert et al., J. Chem. Phys., 1996, 105, 6183]. A parallel study has also been carried out for the carbonyl sulfide-ammonia complex whose pure rotational and ro-vibrational spectra at 6 μm have been detected and analyzed for the first time. The spectral and the subsequent structural analyses, in conjunction with the corresponding ab initio calculation, indicate that the OCS-NH(3) complex assumes C(3v) symmetry with S pointing to N of NH(3), in contrast to the T-shaped geometries obtained for the isoelectronic N(2)O-NH(3) and CO(2)-NH(3) complexes.     

RISM-SCF study of the free-energy profile of the Menshutkin-type reaction NH3+CH3Cl→NH3CH3++Cl− in aqueous solution

The free-energy profile for the Menshutkin-type reaction NH₃ + CH₃Cl → NH₃CH₃ ⁺ + Cl⁻ in aqueous solution is studied using the RISM-SCF method. The effect of electron correlation on the free-energy profile is estimated by the RISM-MP2 method at the HF optimized geometries along the reaction coordinate. Solvation was found to have a large influence on the vibrational frequencies at the reactant, transition state and product; these vibrational frequencies are utilized to calculate the zero-point energy correction of the free-energy profile. The computed barrier height and reaction exothermicity are in reasonable agreement with those of experiment and previous calculations. The change of solvation structure along the reaction path is represented by radial distribution functions between solute-solvent atomic sites. The mechanisms of the reaction are discussed from the view points of solute electronic and solvation structures. Received: 26 June 1998/Accepted: 28 August 1998 / Published online: 2 November 1998     

Ab initio study of the cooperativity between NH···N and NH···C hydrogen bonds in H3N–HNC–HNC complex

Ab initio calculations at the MP2/aug-cc-pVTZ level have been performed to study the cooperativity of hydrogen bonds in homoclusters (HNC–HNC–HNC and HNC–HNC–HNC–HNC) and heteroclusters (H₃N–HNC–HNC and H₃N–HNC–HNC–HNC). The cooperative energies in the HNC–HNC–HNC and H₃N–HNC–HNC trimers are –2.05 and –2.56 kcal/mol, respectively. The result shows that the cooperativity in the heterotrimer is larger than that in the homotrimer. A similar result also happens in the tetramers. The energy decomposition scheme indicates that orbital interaction is a major contribution to the cooperative energy of N···HN hydrogen bond, whereas the electrostatic and orbital interactions to that of C···HN hydrogen bond. The effect of HNC chain length on the strength of N···HN hydrogen bond has also been considered at the MP2/aug-cc-pVDZ level. It is indicated that the interaction energy of N···HN hydrogen bond trends to be a fixed value when the HNC number tends to be infinite, and the strength of N···HN hydrogen bond is regulated mainly through the electrostatic and polarization interactions although the charge transfer interaction also has an effect on it.     

Chirality recognition in the glycidol···propylene oxide complex: a rotational spectroscopic study

Chirality recognition in the hydrogen-bonded glycidol···propylene oxide complex has been studied by using rotational spectroscopy and ab initio calculations. An extensive conformational search has been performed for this binary adduct at the MP2/6-311++G(d,p) level of theory and a total of 28 homo- and heterochiral conformers were identified. The eight binary conformers, built of the two dominant glycidol monomeric conformers, g-G+ and g+G-, were predicted to be the most stable ones. Jet-cooled rotational spectra of six out of the eight conformers were observed and unambiguously assigned for the first time. The experimental stability ordering has been obtained and compared with the ab initio predictions. The relative stability of the two dominant glycidol monomeric conformers is reversed in some cases when binding to propylene oxide. The contributions of monomeric energy, deformation energy, and binary intermolecular interaction energy to the relative stability of the binary conformers are discussed.     

Communication: Competition between π···π interaction and halogen bond in solution: a combined 13C NMR and density functional theory study

Competition between π···π interaction and halogen bond in solution has been investigated by using carbon nuclear magnetic resonance spectroscopy ((13)C NMR) combined with density functional theory calculation. Both experimental and theoretical results clearly show that there are no C-Cl···π or C-Br···π halogen bonds and only the π···π interactions exist in the binary liquid mixtures of C(6)D(6) with C(6)F(5)Cl and C(6)F(5)Br, respectively. The case is totally different for the binary liquid mixtures of C(6)D(6) with C(6)F(5)I in which the C-I···π halogen bonds not the π···π interactions are present. The important role of entropy in the competition between π···π interaction and halogen bond in solution was also discussed.     

Radical reaction HCNO + 3NH: a mechanistic study

The complex triplet potential energy surface for the reaction of HCNO with NH is investigated at the G3B3 level using the B3LYP/6-311++G(d,p), and QCISD/6-311++G(d,p) geometries. Various possible isomerization and dissociation pathways are probed. The initial association between HCNO and NH is found to be carbon to nitrogen attack leading to HNCHNO 2a, which can convert to 2b, 2c, and 2d. Subsequently, 1,4-H-shift of 2a to form NCHNOH 3a followed by dissociation to P ₂ (¹HCN + ³HON) is the most feasible pathway. Much less competitively, 2d undergoes successive 1,3-H-shift and C-N cleavage to form HNCNOH 8b, and then to product P ₃ (¹HNC + ³HON), the second feasible pathway. 8b can alternatively isomerize to 8c followed by N–O bond rupture to generate P ₆ (²OH + ²HNCN), the lesser followed feasible pathway. In addition, 2b takes continuously 1,3- and 1,2-H-shift to form NC(H)NHO 6a, then to ONHCNH 7a which can convert to 7b. Eventually, 7b may take C-N bond fission to produce P ₅ (¹HNC + ³HNO), the least feasible pathway. The present paper may be helpful for future experimental identification of the product distributions for the title reaction, and may be helpful to deeply understand the mechanism of the title reaction.     

Experimental and theoretical study of the dissociation enthalpy of the N–O bond on 2-hydroxypyridine N-oxide: theoretical analysis of the energetics of the N–O bond for hydroxypyrydine N-oxide isomers

The standard (p^∘=0.1 MPa) molar enthalpy of formation of crystalline 2-hydroxypyridine N-oxide was measured, at T=298.15 K, by static bomb calorimetry and the standard molar enthalpy of sublimation, at T=298.15 K, was obtained using Calvet microcalorimetry. These values were used to derive the standard molar enthalpy of formation of 2-hydroxypyridine N-oxide in gaseous phase, and to evaluate the dissociation enthalpy of the N–O bond. Additionally, high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional have been performed for the three isomers of hydroxypyridine N-oxide in order to confirm the experimental trend for the dissociation enthalpy of the (N–O) bond.     

Bond distances and bond angles of the C10 skeleton in naphthalene derivatives as hard bond parameters:35Cl NQR and structure of 1,8-diaminonaphthalene · Cl2HCCOOH, 1,8-diaminonaphthalene · Cl3CCOOH, 1-aminonaphthalene · Cl3CCOOH,and 1,8-diaminonaphthalene

The structures of several naphthalene derivatives and their³⁵Cl NQR spectra have been investigated. 1,8-Diaminonaphthalene,C _2v ⁹ -Pna2 ₁, Z = 8,a (in pm) = 881,b = 1577,c = 1213; 1,8-diaminonaphthalene monodichloroacetate,C _2h ⁶ -C2/c, Z = 8,a = 2050,b = 584,c = 2333, (in degrees) = 110.1; 1,8-diaminonaphthalene monotrichloroacetate,C ₁ ¹ -P¯1, Z=2,a=1211,b=1062,c=589,=74.8,=80.1,=70.9; 1-aminonaphthalene trichloroacetate,D _2h ¹⁵ -Pbca, Z=8,a=2347,b=1289,c=889. The³⁵Cl NQR spectrum of 1,8-diaminonaphthalene monodichloroacetate is a doublet, the frequencies decreasing with increasing temperature from 77 to 217 K at which temperatureT _b the NQR signals bleach out. A³⁵Cl NQR triplet is found for 1,8-diaminonaphthalene monotrichloracetate in the range 77 77K 207 (=T _b ). 1-Amino-naphthalene trichloroacetate shows a³⁵Cl NQR triplet, too, withT _b = 136 K. Characteristic for the intermolecular interactions are hydrogen bonds in the chloroacetic acid salts; each NH₃ group forms three hydrogen bonds, and of the two oxygens one is involved in two such bonds, one in one bond. Thereby units of two cations and two anions are formed, and these dirners are connected to strings by hydrogen bonds. Additional van der Waals interactions between the chlorine atoms and the naphthalene ring system are observed. Comparison of the intramolecular bond distances C(i)-C(j) of the C₁₀ naphthalene skeleton for 41 naphthalene derivatives (present data and literature) shows that the bond distances C(i)-C(j)are little influenced by substitution, as is the mean bond length. Shorter and longer distances prove a partial localization of charge at C(1)-C(2), C(3)-C(4), C(5)-C(6), and C(7)-C(8). Regularities within the bond angles and characteristic influences of 1,8-disubstitution on it are discussed.     

On the mechanism for the photooxidation of aromatic azides containing a secondary N–H bond: A sequence of intramolecular transformations with the formation of heterocyclic oximes

During the photooxidation of aromatic azides containing a secondary N–H bond at the para-position, a sequence of intramolecular transformations of nitroso oxides led to the formation of heterocyclic oximes along with the corresponding nitroso and nitro compounds.     

C-X···π halogen and C-H···π hydrogen bonding: interactions of CF3X (X = Cl, Br, I or H) with ethene and propene

Using FTIR and Raman spectroscopy, the formation of halogen bonded complexes of the trifluorohalomethanes CF(3)Cl, CF(3)Br and CF(3)I with ethene and propene dissolved in liquid argon has been investigated. For CF(3)Br and CF(3)I, evidence was found for the formation of C-X···π halogen bonded 1:1 complexes. At a higher ratio of CF(3)I/propene, weak absorptions due to a 2:1 complex were also observed. Using spectra recorded at different temperatures, the complexation enthalpies for the complexes were determined to be -5.3(2) kJ mol(-1) for CF(3)Br·ethene, -7.5(2) kJ mol(-1) for CF(3)I·ethene, -5.6(1) kJ mol(-1) for CF(3)Br·propene, -8.8(1) kJ mol(-1) for CF(3)I·propene and -16.5(6) kJ mol(-1) for (CF(3)I·)(2)propene. The complexation enthalpies of the hydrogen bonded counterparts, with CF(3)H as the Lewis acid, were determined to be -4.6(4) kJ mol(-1) for CF(3)H·ethene and -5.1(2) kJ mol(-1) for CF(3)H·propene. For both hydrogen bonded complexes, a blue shift, by +4.8 and +4.0 cm(-1), respectively, was observed for the C-H stretching mode. The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level.     

Unimolecular reactions in the CF3CH2Cl ↔ CF2ClCH2F system: isomerization by interchange of Cl and F atoms

The recombination of CF(2)Cl and CH(2)F radicals was used to prepare CF(2)ClCH(2)F* molecules with 93 ± 2 kcal mol(-1) of vibrational energy in a room temperature bath gas. The observed unimolecular reactions in order of relative importance were: (1) 1,2-ClH elimination to give CF(2)═CHF, (2) isomerization to CF(3)CH(2)Cl by the interchange of F and Cl atoms and (3) 1,2-FH elimination to give E- and Z-CFCl═CHF. Since the isomerization reaction is 12 kcal mol(-1) exothermic, the CF(3)CH(2)Cl* molecules have 105 kcal mol(-1) of internal energy and they can eliminate HF to give CF(2)═CHCl, decompose by rupture of the C-Cl bond, or isomerize back to CF(2)ClCH(2)F. These data, which provide experimental rate constants, are combined with previously published results for chemically activated CF(3)CH(2)Cl* formed by the recombination of CF(3) and CH(2)Cl radicals to provide a comprehensive view of the CF(3)CH(2)Cl* ? CF(2)ClCH(2)F* unimolecular reaction system. The experimental rate constants are matched to calculated statistical rate constants to assign threshold energies for the observed reactions. The models for the molecules and transition states needed for the rate constant calculations were obtained from electronic structures calculated from density functional theory. The previously proposed explanation for the formation of CF(2)═CHF in thermal and infrared multiphoton excitation studies of CF(3)CH(2)Cl, which was 2,2-HCl elimination from CF(3)CH(2)Cl followed by migration of the F atom in CF(3)CH, should be replaced by the Cl/F interchange reaction followed by a conventional 1,2-ClH elimination from CF(2)ClCH(2)F. The unimolecular reactions are augmented by free-radical chemistry initiated by reactions of Cl and F atoms in the thermal decomposition of CF(3)CH(2)Cl and CF(2)ClCH(2)F.     

Investigation of rotational isomerism along the C-O bond in α-aryloxycarbonyl compounds

Electrical and electrooptical characteristics of -phenoxycyclohexanone, -phenoxyacetone,p-Me-,p-Cl-,p-Br-,p-NO ₂ -phenoxycyclohexanones, andp-NO ₂ -phenoxyacetone in the solution in CCl ₄ were determined; these were utilized for the conformational analysis of the general structural fragment Ar-O-C-C=O. The presence of conformers with theac- andsp-structure at the C _sp 3-C _sp 2 bond was established in the equilibrium. The content of thesp-form increases with the increase in the polar character of the substituent at thep-position of the benzene ring. Thegauche isomer (sp-G orsp-G) is realized along the C-O bond in it. In theac-conformers, both thegauche (ac-G) and the trans (ac-T) structures can be realized along the C-O bond.A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian Academy of Sciences, Kazan' 420083. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 612–620, March, 1992.     

Theoretical study of the C�CF bond activation in methyl fluoride by alkaline-earth metal monocations

Reactions of methyl fluoride with bare alkaline-earth metal monocations (Mg⁺, Ca⁺, Sr⁺, and Ba⁺) were studied using theoretical methods. Thermochemical data were calculated using density functional theory in conjunction with polarized 3-?? and 4-?? basis sets. Variational/conventional microcanonical transition state theory was used for the calculation of the reaction rate constants over a large range of temperatures. According to our calculations, the Ca⁺, Sr⁺, and Ba⁺ reactions with CH₃F proceed to yield CaF⁺, SrF⁺, and BaF⁺, in agreement with the experimental observation. The theoretically predicted global rate constants are in reasonable agreement with the experimental data. In the case of Mg⁺, the large value of the computed energy barrier associated with the ??inner?? transition structure is fully consistent with the limited progress experimentally observed for this reaction. The importance of bottlenecks other than the ??inner?? transition state is highlighted and its mechanistic implications discussed. Particularly, our calculations suggest that the studied processes proceed through a ??harpoon-like?? mechanism.     

High-resolution FTIR study of the 3ν1, rovibrational band of 16O14N35Cl

818 A type (ΔK_a = 0) rovibrational lines of the near-prolate asymmetric top ¹⁶O¹⁴N³⁵Cl have been assigned in the 3v₁