Effect of the carbon atom coordination on the thermoactivated motion of the carbon-bonded trichloromethyl group

The reorientational motion of the trichloromethyl group depending on the environment of the carbon atom is considered based on the chlorine-35 nuclear quadrupole resonance (NQR) data. The study is performed for CCl₃ groups bonded to tri- and tetracoordinated carbon atoms in the crystalline compounds CCl₃CCl=NR (R = CH₂C₆H₅ and CCl₃CCl₃) and CCl₃CXClN=CClC₆H₄NO_2-n (X = H and Cl).₃₅Cl NQR studies of thermoactivated motion in 70 solid trichloromethyl-containing compounds are summarized. The ranges of activation energies of CCl₃ reorientations at tri- and tetracoordinated carbon atoms were determined to be 10-50 and 30-90 kJ/mole, respectively, the activation energy being markedly greater in the latter case. CCl₃ reorientations may be completely frozen by the bulky substituents which may be present along with CCl₃ in the tetracoordinated carbon bonds. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 4, pp. 737-746, July-August, 2000     

Potential barrier to CCl3 reorientations in the CCl3 CXClN=CClC6H4NO2-n compounds (X = H and Cl)

The factors hindering the reorientational motion of the CCl₃ group in the isolated CCl₃CXClN=CClC₆H₄NO₂-p molecules with X = H and Cl were analyzed based on RHF/6-31G* calculations. Using the published ³⁵Cl NQR data, the intramolecular and lattice contributions to the potential barrier of CCl₃ reorientations were compared. The character and magnitude of the hindrances responsible for the barriers to reorientations in these compounds are discussed.     

35Cl NQR and structural-dynamic properties of crystalline CCl3C(O)OC6H2Cl3-2,4,6

The temperature dependences of the resonance frequency and quadrupole spin-lattice relaxation time of the chlorine-35 nuclei in crystalline CCl₃C(O)OC₆H₂Cl₃-2,4,6 were studied. Owing to the presence of resonant nuclei in various fragments of the molecules, the character of dynamics of these molecular fragments and the entire molecule was established. It is shown that thermal librations of molecules are quasiharmonic over the whole temperature range from 77 K to the melting point of the crystal. The reorientational motion of the CCl₃ group bonded to the three-coordinated carbon atom was revealed. This motion causes an exponential increase in the spin-lattice relaxation time of the chlorine nuclei of this group and subsequent damping of NQR signals (chlorine-35 resonance signals of the aryl radical were observed before melting of the sample). The activation energy of the reorientational motion is found; its value, which is 27.3 kJ/mole, is considered in comparison with the activation energies obtained by the Cl NQR method in solids for CCl₃ reorientations in similar molecular structures.     

Structure of the CCl3CCl=NCH2C6H5 molecule and reorientations of its CCl3 group, according to ab initio calculations

The optimal geometry of the isolated CCl₃CCl=NCH₂C₆H₅ molecule and the intramolecular barrier to reorientations of its trichloromethyl group are calculated by the RHF/6-31G* and B3LYP/6-31G* methods. The barrier found (14.1 kJ mol^?1) is compared to that determined previously by ³⁵Cl NQR for a crystal of this compound, which allows estimation of the contribution of intermolecular interactions to braking of this motion of the CCl₃ group. The structural features of the molecule of this compound are consistently manifested in quantum-chemical calculations and NQR spectra.     

35Cl NQR study of CCl3 reorientations in the carbonyl-containing derivatives of 3,3,3-trichloropropene

Temperature dependences of³⁵Cl NQR frequencies and spin-lattice relaxation times were measured for the CCl₃ groups of compounds CCl₃CH=CHR and CCl₃CH=CR₂ with R=COCH₃ and COOCH₃. The activation energies of CCl₃ reorientations reflect the significant contribution of intermolecular steric interactions to the value of the potential barrier hindering these reorientations. In the CCl₃CH=CHCOCH₃ crystal, the reorientations of the CCl₃ group are hindered more than those of the molecule as a whole. Perm State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 2, pp. 54–60, March–April, 1994. Translated by L. Smolina     

Intramolecular Hindrances to CCl3 Reorientations in CCl3PCl4 and (CCl3)2PCl3 Crystals

The trigonal bipyramidal structure of trichloromethyl-tetrachlorophosphorane CCl₃PCl₄ and bis(trichloromethyl)trichlorophosphorane (CCl₃)₂PCl₃ molecules with apical CCl₃ groups is responsible for steric hindrances to reorientations of these groups around the C–P bond. The intramolecular barriers to CCl₃ reorientations calculated for the two phosphoranes (Hartree–Fock method using the 6-31G(d) basis set) are 105.9 kJ/mole and 106.6 kJ/mole, respectively. These internal barriers are high enough to block the reorientational motion of the CCl₃ groups, as found previously by ³⁵Cl NQR for the stated crystals.     

Molecular structure of trichlorophosphazo compounds from ab initio calculation and chlorine-35 NQR spectroscopy data

The molecular structures of trichlorophosphazo compounds Cl₃P=NR with R = C(CH₃)₃, C(C₂H₅)₃, C(CF₃)₃, CCl(CF₃)₂, CCl₂CF₃, CCl₃, CCl₂CCl₃, and CCl(CCl₃)₂ were analyzed by combined consideration of the results of ab initio MP2/6-31G* calculations and previously measured ³⁵Cl NQR frequencies. The conformational peculiarity of these molecules caused by the relative spatial orientations of the P-Cl and N-C bonds is reflected in the calculated geometric parameters of the Cl₃P=NC fragments and in the pattern of ³⁵Cl quadrupole resonance spectra for PCl₃ groups. For these atomic groups, the ³⁵Cl NQR frequencies were brought in correlation with the charges of the chlorine atoms found by quantum-chemical calculations.     

The activation energy of CCl<Subscript>3</Subscript> group reorientation motion in crystals estimated from the NQR data

The possibility of estimating the activation energy E _a of reorientation of symmetrical atomic groups containing quadrupole nuclei from nuclear quadrupole resonance data was studied. The estimates can be obtained using simple semiempirical equations relating the E _a value to the characteristic temperature selected in some general way for different compounds; the measured NQR parameters are determined at this temperature by reorientation motion. The temperature at which the spin-lattice relaxation time of a moving quadrupole nucleus is 1 ms and the fade out temperature of the NQR signal for the group that experiences reorientation (fading is related to the maximum measurable width of resonance lines) were tested as characteristic temperatures. The ³⁵Cl NQR data on 70 reorienting CCl₃ groups were analyzed to obtain quantitative relations of the E _a = CT _char type that described the reorientations of these groups in solids. Some corollaries to the equations obtained are discussed.     

35Cl NQR spectra, structure, and mutual influence of substituents in the series of chlorine-containing organic compounds of pentavalent phosphorus

Regularities of transmission of the effect of equatorial and axial substituents were established for a number of chlorine-containing organic compounds of pentatalent phosphorus using³⁵Cl NQR spectra. The field constants of³⁵Cl NQR frequencies of the chlorine atoms participating in the P−Cl bond were estimated for a series of tetrahedral phosphorus ions. The³⁵Cl NQR frequencies of chlorine atoms in tetracoordinated ions of pentavalent phosphorus and in analogous isoelectronic silicon compounds are related by a linear dependence. Deceased Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 915–922, May, 1999.     

Thermally activated intramolecular motion in the crystalline complex SbCl3 · C6H4(NO2)2-m

Temperature dependences of the ³⁵Cl and ¹²¹Sb NQR frequencies and the spin-lattice relaxation times in the molecular complex of antimony trichloride with meta-dinitrobenzene were investigated. These dependences for ³⁵Cl nuclei suggest a thermoactivated motion of chlorine atoms with the activation energy E _a = 50 kJ/mol in the crystal. The data obtained were used for determination of the molecular structure of the complex. Some features of the intramolecular motion in SbCl₃ complexes of the vπ and vn types were discussed. The ¹²¹Sb NQR data revealed no thermoactivated motion of the ligands bound to the Sb atom.     

Quantum chemical calculations and35Cl NQR study of the molecular structure of trichloroacetyl halides and trichloroacetaldehyde

An optimal geometry of CCl₃C(O) X molecules (X=H, F, Cl, Br, and I) is determined by the MNDO quantum chemical method. According to the results of calculations, the molecules in the free state have an eclipsed conformation with a syn-peri-planar position of the oxygen atom and one of the chlorine atoms of the trichloromethyl group. However, as follows from the³⁵Cl NQR spectra at different temperatures, in the crystal state of CCl₃C(O)X this conformation is distorted under the influence of intermolecular interactions due to the torsional rotation of the CCl₃ group around the C-C bond. Indicatory potentialities of the NQR method with respect to the structural features that are due to crystallographic effects are discussed. Perm State University. Translated fromZhurnal Strukturmoi Khimii, Vol. 37, No. 3, pp. 494–500, May-June, 1996.     

Specific Interactions and the Nature of the Chemical Nonequivalence of Chlorine Atoms in the ECl<Subscript>3</Subscript> Group (E = C,P) of Trichlorophosphonium Ylides and Isomeric Dichloro(trichloromethyl)phosphine

The nature of the asymmetry of the ECl₃ group (E = C, P) in dichloro(trichloromethyl)phosphine and trichlorophosphonium methylide and dichloromethylide was studied by nonempirical and MNDO-PM3 calculations and ³⁵Cl NQR spectroscopy. The chemical nonequivalence of chlorine atoms in the CCl₃ group of dichloro(trichloromethyl)phosphine is caused by steric interaction with the PCl₂ fragment. In the PCl₃ group of trichlorophosphonium methylide and dichloromethylide, this nonequivalence arises from specific interactions with the lone electron pair of the ylide carbon atom, whose p-character is about 94% on the RHF/6–311⁺⁺G(3df,3pd) level. The alteration of the type of interaction predicts inversion of the ³⁵Cl NQR signals of the PCl₃ group in trichlorophosphonium ylide and dichloromethylide in relation to those of the CCl₃ group in the experimental spectrum of dichloro(trichloromethyl)phosphine. The MNDO-PM3 method characterizes dichloro- and diphenylmethylides as unstabilized structurally stable ylides.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 7, 2005, pp. 1148–1153.Original Russian Text Copyright © 2005 by Romanenko.     

An ab initio study of the electronic and steric structure of Cl<Subscript>2</Subscript>ZX molecules (Z = P and As)

The electronic and steric structure of the Cl₂ZX molecules [Z = P and As, X = C₂H₅, N(CH₃)₂, and OCH₃] was examined by RHF/6-31G(d) and MP2/6-31G(d) calculations. The data on the electron distribution at the Cl atoms are compared with the published ³⁵Cl NQR data. The main reason for a decrease in the NQR frequency of the molecules with X = N(CH₃)₂ and OCH₃ as compared to the ethyl-substituted compounds is an increase in the population of the 3p components of their p _z(p _σ) orbitals. With X = N(CH₃)₂, electron distribution at two Cl atoms differs significantly.     

Features of the molecular structure of compounds of (Cl3PNAr)2 series (Ar = C6H4Z, Z = H, Cl) by quantum-chemical calculations and the data of 35Cl nuclear quadrupole resonance

A series of dimeric molecules (Cl₃PNAr)₂ where Ar = C₆H₄Z, Z = H or Cl in 2–4 positions, was calculated by the RHF/6-31G* method with the full geometry optimization. A specific nature of the intramolecular steric orientation of aryl groups was revealed, therewith in the case Ar = C₆H₄Cl-2 short nonvalent contacts were founf between the ortho-chlorine atom and PCl₃ group manifested in the peculiarity of geometric parameters and NQR spectroscopy data of this compound. The correlation of the ³⁵Cl NQR frequencies of P-Cl bonds with the values of the charges on the chlorine atoms obtained by quantum-chemical calculations of the molecules was analyzed.     

Comparison of molecular structure and 35Cl nuclear quadrupole coupling tensors in 2,3,6-trichlorophenyl acetate

The crystal structure of 2,3,6-Cl₃C₆H₂OCOCH₃ was determined at room temperature; C⁵_2h-P2₁/c; Z=4. ³⁵Cl Zeeman split NQR at 294K shows, that the EFG tensors are mainly determined by the CCl bonds. φ_zz for the three Cl atoms is nearly parallel to the directions CCl. The asymmetry parameter of the EFG's are 0.1423 for Cl(2), 0.1037 for Cl(3) and 0.1015 for Cl(6). φ_xx of Cl(2), Cl(3) and Cl(6) is almost perpendicular to the ring plane. The NQR results are discussed with respect to the molecular structure. Temperature dependences of ³⁵Cl NQR resonances are reported for the range 77≦ T/K≦ 300.     

Ab initio calculations of of pyridine and its 2- and 3-chloroderivatives

Ab initio calculations of the C₅H₅N, 2-. and 3-ClC₅H₄N molecules by the RHF method in the valence split 6-31G^* basis set with full optimization of the geometry have been carried out. The alternation of the charges on the atoms of the pyridine ring and of the populations of their p _x -orbitals is in agreement with the noninductive through -the-field interaction of the geminal atoms. The³⁵Cl NQR frequencies and the electric field gradient asymmetry parameters at the³⁵Cl nuclei in 2- and 3-CIC₅H₄N were estimated using the populations of the valentp-orbitals of the Cl atoms and their components. The³⁵Cl NQR frequency for the first compound is lower than that for the second one, mainly due to the higher p-electron population of its Cl atom.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2641–2644, November, 1996.     

Quantum-chemical study of chlorophosphoranes: I. Structure of the (C<Subscript>6</Subscript>H<Subscript>5</Subscript>)<Subscript>2</Subscript>PCl<Subscript>3</Subscript> molecule in gaseous and crystalline states

The quantum-chemical calculations of the (C₆H₅)₂PCl₃ molecule were performed by the methods RHF/6-31G(d) and MP2/6-31G(d) with complete and partial optimization of its geometry. The results of calculations were used for evaluating the NQR frequencies of ³⁵Cl and the parameters of asymmetry of the of electric field gradient on the ³⁵Cl nuclei of this molecule. According to the results of calculations with the partial optimization of geometry and to the experimental data of ³⁵Cl NQR the bond lengths were found of central atom P in the solid state of this compound. The mutual influence of its axial and equatorial bonds was studied. It is shown that in chlorophosphoranes the ³⁵Cl NQR frequencies of the axial and equatorial chlorine atoms decrease with the decrease in the bond lengths of phosphorus atom.     

Quantum-chemical study of chlorophosphoranes: II. Structure of molecules of (Cl2)aP(ClnX3?n)e series, and mutual influence of axial and equatorial substituents

The quantum-chemical calculations of the (C₆H₅)₂PCl₃ molecule were performed by the methods RHF/6-31G(d) and MP2/6-31G(d) with complete and partial optimization of its geometry. The results of calculations were used for evaluating the NQR frequencies of ³⁵Cl and the parameters of asymmetry of the of electric field gradient on the ³⁵Cl nuclei of this molecule. According to the results of calculations with the partial optimization of geometry and to the experimental data of ³⁵Cl NQR the bond lengths were found of central atom P in the solid state of this compound. The mutual influence of its axial and equatorial bonds was studied. It is shown that in chlorophosphoranes the ³⁵Cl NQR frequencies of the axial and equatorial chlorine atoms decrease with the decrease in the bond lengths of phosphorus atom.     

35Cl Nuclear Quadrupole Resonance and Thermally Activated Molecular Motion in the 2:1 Crystalline Complex of Antimony Trichloride with Benzene

Nuclear quadrupole resonance (NQR) spectra of chlorine and antimony in the 2SbCl₃ · C₆H₆ complex and their temperature behavior between 77 K and the melting point were studied. The spectral lines of two nonequivalent SbCl₃ moieties are compared with available Xray diffraction data. An analysis of the temperature dependence of the resonant frequency and the spin–lattice relaxation time for ³⁵Cl nuclei showed the existence of thermally activated, intramolecular motion of chlorine atoms in this complex. The nature of the motion is determined by the shape of the SbCl₃ moieties. The activation energies of the motion were obtained for the two structurally nonequivalent halves of the complex (75.5 and 79.5 kJ/mole).     

Electronic Effects in X(CH2)nY Homologous Series by NMR and Quantum Chemistry

The ^79,81Br NQR spectra of compounds of the Br(CH₂) _n COOH (n = 1-5) and Br(CH₂) _n COOSi·(CH₃)₃ (n = 1, 2, 4, 5) homologous series were measured at 77 K. The NQR frequencies of compounds of the first series as n increases to 3 and then oscillate. The ³⁵Cl NQR frequencies of Cl(CH₂) _n Cl series molecules, estimated from the Cl3p populations resulting from RHF/6-31G(d) calculations, steadily decrease as n increases from 1 to 10. No oscillation of calculated ³⁵Cl frequencies was revealed for compounds of the latter series (by contrast to experimental observations). The calculation results give no way to deducing the mechanism of the oscillation effect.