Synthesis of dragmacidin D via direct C-H couplings

Dragmacidin D, an emerging biologically active marine natural product, has attracted attention as a lead compound for treating Parkinson's and Alzheimer's diseases. Prominent structural features of this compound are the two indole-pyrazinone bonds and the presence of a polar aminoimidazole unit. We have established a concise total synthesis of dragmacidin D using direct C-H coupling reactions. Methodological developments include (i) Pd-catalyzed thiophene-indole C-H/C-I coupling, (ii) Pd-catalyzed indole-pyrazine N-oxide C-H/C-H coupling, and (iii) acid-catalyzed indole-pyrazinone C-H/C-H coupling. These regioselective catalytic C-H couplings enabled us to rapidly assemble simple building blocks to construct the core structure of dragmacidin D in a step-economical fashion.     

Interactions of D2O with methane and fluoromethane surfaces

TOF-SIMS is used to investigate the interactions between D2O and hydrophobic molecules, such as CH4, CH3F, CH2F2, CHF3, and CF4, at cryogenic temperatures (15 K). By irradiation with a 1.5-keV He+ beam, the D(+)(D2O)n ions are ejected efficiently from the D2O nanoclusters physisorbed on the CF4 layer due to Coulomb explosion: the ion yields are by about two orders of magnitude higher than those from a thick D2O layer via the kinetic sputtering. The D(+)(D2O)n yields decrease on the CHnF(4-n) layer with increasing the number of the C-H group. This is because the Coulombic fission is quenched due to the delocalization of valence holes through the C-H...H-C and C-H...D2O contacts. A pure D2O film is hardly grown on the CH4 layer as a consequence of intermixing whereas the D2O molecules basically adsorb on the surfaces of fluoromethanes, suggesting the attractive (water-repellent) interactions in the C-H...D2O (C-F...D2O) contacts. The C-H...O bond behaves like a conventional O-H...O hydrogen bond as far as the collision-induced proton transfer reaction is concerned.     

An oxidative cross-dehydrogenative-coupling reaction in water using molecular oxygen as the oxidant: vanadium catalyzed indolation of tetrahydroisoquinolines

An aerobic oxidative cross-dehydrogenative coupling reaction between sp(3) C-H and sp(2) C-H bonds is developed by employing a vanadium catalyst (10 mol%) in an aqueous medium using molecular oxygen as the oxidant. This environmentally benign strategy exhibits larger substrate scope and shows high regioselectivity.     

Aromatic vs aliphatic C-H bond activation by rhodium(I) as a function of agostic interactions: catalytic H/D exchange between olefins and methanol or water

The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)(2)(solv)(n)()]BF(4) (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a). acetone, (b). THF, (c). methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through eta(2)-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.     

Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage

An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.     

Palladium-catalyzed alkylation of aryl C-H bonds with sp3 organotin reagents using benzoquinone as a crucial promoter

The combination of directed C-H activation, batch-wise addition of tetraalkyltin reagents, and rate enhancement by benzoquinone and microwave irradiation provides a promising strategy for the direct coupling of C-H bonds with organometallic reagents. A variety of tetraalkyltins were coupled to C-H bonds to give the alkylated products in good yields by using 5 mol % Pd(OAc)2 as the catalyst. Benzoquinone was shown to be essential for the C-H activation when substrates containing non-pi-conjugated chelating groups are used. Monitoring the formation and reductive elimination of the Pd(Ar)(Me)L2 complex also revealed that benzoquinone promotes the reductive elimination step. Microwave irradiation enhances the reaction rate drastically. The versatility of this protocol was demonstrated by using substrates containing either oxazoline or pyridine as directing groups.     

Structural organization in aqueous solutions of 1-Butyl-3-methylimidazolium halides: a high-pressure infrared spectroscopic study on ionic liquids

High-pressure infrared spectroscopy was applied to study the hydrogen-bonding structures of 1-butyl-3-methylimidazolium halides/D2O mixtures. No drastic changes were observed in the concentration dependence of the alkyl C-H band frequency at high concentration of 1-butyl-3-methylimidazolium chloride. Nevertheless, the alkyl C-H exhibits an increase in frequency upon dilution at low concentration. These observations may indicate a clustering of the alkyl groups at high concentration and the formation of a certain water structure around alkyl C-H groups in the water-rich region. The imidazolium C-H band at ca. 3051 cm(-1) displays a monotonic blue-shift in frequency as the sample was diluted at high concentration of 1-butyl-3-methylimidazolium chloride. That is, water can be added to change the structural organization of 1-butyl-3-methylimidazolium chloride in the ionic liquid-rich composition region by introducing water-imidazolium C-H interactions. Analyzing the pressure dependence of the imidazolium C-H stretches yielded anomalous nonmonotonic pressure-induced frequency shifts. This result may reflect the strengthening of C-H-O interactions between imidazolium C-H groups and the water clusters. Density functional theory calculations also revealed that the characteristic bonded C2-H vibration may be shifted via the modification of C2-H-Cl- associations.     

High selectivity for primary C-H bond cleavage of propane σ-complexes on the PdO(101) surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.     

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity.     

Silver(I) complexes in coordination supramolecular system with bulky acridine-based ligands: syntheses, crystal structures, and theoretical investigations on C-H...Ag close interaction

In our efforts to investigate the coordination architectures of transition metals and organic ligands with tailored structures, we have prepared two structurally related rigid bulky acridine-based ligands, 9-[3-(2-pyridyl)pyrazol-1-yl]- acridine (L(1)) and 9-(1-imidazolyl)acridine (L2), and synthesized and characterized four of their Ag(I) complexes, {[AgL1](ClO4)}2 (1), {[AgL1](NO3)}2 (2), [AgL2(2)](ClO4) (3), and {[(Ag3L2(3))(NO3)](NO3)2(H2O)}(infinity) (4). The single-crystal X-ray diffraction analysis shows that the structures of 1 and 2 are similar to each other, with the two intramolecular Ag(I) centers of each complex being encircled by two L1 ligands; this forms a unique boxlike cyclic dimer, which is further linked to form one-dimensional (1D) chains of 1 and a two-dimensional (2D) network of 2 by intermolecular face-to-face pi...pi stacking and/or weak C-H...O hydrogen-bonding interactions, respectively. 3 has a mononuclear structure, which is further assembled into a 2D network via intermolecular Ag...O and pi...pi stacking weak interactions. 4 possesses two different 1D motifs that are further interlinked through interlayer face-to-face pi...pi stacking and Ag...O weak interactions, resulting in a 2D network. It is worth noting that one of the interesting structural features of 1, 2, and 4 is the presence of obvious C-H...M hydrogen-bonding interactions between the Ag centers and some acridine ring H atoms identified by X-ray diffraction on the basis of the van der Waals radii. Furthermore, as a representative example, full geometry optimization on the basis of the experimental structure, the natural bond orbital (NBO), and topological analysis of 1 were carried out by DFT and AIM (Atoms in Molecules) calculations. The total C-H...Ag interaction energy in 1 is estimated to be about 14 kJ/mol. Therefore, this work offers three new rare examples (1, 2, and 4) that exhibit C-H...Ag weak interactions, in which the N donors of the acridine rings coordinate to Ag(I) ions. Also, these results strongly support the existence of C-H...Ag close interactions and allow us to have a better understanding of the nature of such interactions in the coordination supramolecular systems.     

Application of long range j c-h resolved 2d spectroscopy (lrjr) in structural elucidation of natural products. The structure of oxirapentyn

The structure of a fungal metabolite, oxyrapentyn has been determined as shown in Fig. 7 by application of a new NMR technique, long range J C-H resolved 2D spectroscopy (LRJR).     

Two one-dimension copper(II) coordination polymers based on imine-based bidentate Schiff-base ligand: synthesis, crystal structure and luminescent properties

Two new one-dimension copper(II) coordination polymers [CuL(2)(NCS)(2)](n) (1) and [CuL(2)(NO(3))(2)](n) (2) (L=(C(5)H(4)N)C(CH(3))=N-N=(CH(3))C(C(5)H(4)N)) have been synthesized and characterized by IR, elemental analysis, TG technique and X-ray crystallography. Each Cu(II) atom has a distorted octahedral N(6) (1) or N(4)O(2) (2) environment with four pyridyl N atoms from four ligands and two N atoms from two NCS(-) anions for polymer 1 or two O atoms from two NO(3)(-) anions for polymer 2, respectively. A pair of bis-monodentate bridging ligands links two Cu(II) centers to form one dimension chain structure containing bimetallic 22-membered macrometallacyclic rings. 1D chain is held together with its neighboring ones via C-H?S hydrogen bonds for 1 and C-H?O hydrogen bonds, C-H?π interactions for 2 to form a 3D supramolecular structure, respectively. The luminescent properties of the polymers 1 and 2 were investigated in the solid state at room temperature.     

Copper(II), cobalt(II), and nickel(II) complexes with a bulky anthracene-based carboxylic ligand: syntheses, crystal structures, and magnetic properties

To systematically explore the influence of the bulky aromatic ring skeleton with a large conjugated pi-system on the structures and properties of their complexes, six CuII, CoII, and NiII complexes with the anthracene-based carboxylic ligand anthracene-9-carboxylic acid (HL1), were synthesized and characterized, sometimes incorporating different auxiliary ligands: [Cu2(L1)4(CH3OH)2](CH3OH) (1), [Cu4(L1)6(L2)4](NO3)2(H2O)2 (2), {[Cu2(L1)4(L3)](CH3OH)0.25}infinity (3), [Co2(L1)4(L4)2(micro-H2O)](CH3OH) (4), {[Co(L1)2(L5)(CH3OH)2]}infinity (5), and {[Ni(L1)2(L5)(CH3OH)2]}infinity (6) (L2 = 2,2'-bipyridine, L3 = 1,4-diazabicyclo[2.2.2]octane, L4 = 1,10-phenanthroline, and L5 = 4,4'-bipyridine). 1 has a dinuclear structure that is further assembled to form a one-dimensional (1D) chain and then a two-dimensional (2D) network by the C-H...O H-bonding and pi...pi stacking interactions jointly. 2 takes a tetranuclear structure due to the existence of the chelating L2 ligand. 3 possesses a 1D chain structure by incorporating the related auxiliary ligand L3, which is further interlinked via interchain pi...pi stacking, resulting in a three-dimensional (3D) network. 4 also has a dinuclear structure and then forms a higher-dimensional supramolecular network through intermolecular pi...pi stacking and/or C-H...pi interactions. 5 and 6 are isostructural complexes, except they involve different metal ions, showing 1D chain structures, which are also assembled into 2D networks from the different crystallographic directions by interchain pi...pi stacking and C-H...pi interactions, respectively. The results reveal that the steric bulk of the anthracene ring in HL1 plays an important role in the formation of 1-6. The magnetic properties of the complexes were investigated, and the very long intermetallic distances result in weak magnetic coupling, with the exception of 1 and 3, which adopt the typical paddle-wheel structure of copper acetate and are thus strongly coupled.     

2-[(2-溴苯胺基)甲基]-4-氯苯酚Schiff碱及其铜配合物的合成及晶体结构

以5-氯水杨醛和邻溴苯胺为原料合成了一种新的Schiff碱配体2-[(2-溴苯胺基)甲基]-4-氯苯酚(1)(C₁₃H₉BrClNO,H₂L),继而与过渡金属铜离子配合,得到其配合物2 ([Cu(C₁₃H₈BrClNO)₂],CuL₂)。通过X-射线衍射法对配体及其配合物进行了结构表征。化合物1属正交晶系,Pbca空间群,晶胞参数a=0.710 19(12) nm,b=1.308 2(2) nm,c=2.533 3(5) nm,M_r=310.57,V=2.353 6(7) nm³,D_c=1.753 g·cm^-3,Z=8,μ=3.700 mm^-1,Z=8,F(000)=1 232,R=0.025 0,wR=0.055 5;化合物1依靠分子间的C-H…N,C-H…O,C-H…Cl 氢键及π-π堆积作用进一步联结成二维网状结构。化合物2属单斜晶系,P2₁/c空间群,晶胞参数a=0.956 8(2) nm,b= 1.085 3(3) nm,c=1.204 7(3) nm,β=105.965(7)°,M_r=682.67,V=1.202 8(6) nm³,D_c=1.885 g·cm^-3,Z=2,μ=4.481 mm^-1,F(000)=670,R=0.045 0,wR=0.122 5。2依靠分子间C-H…π作用及卤素…卤素作用进一步联结成三维网状结构。     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

对氯苄基三苯基镌-马来二氰基二硫烯镍配合物的合成和晶体结构

合成了一种新的含取代苄基三苯基镌的马来二氰基二硫烯镍配合物[ClBzTPP]2[M（mnt）2]·H2O（[ClBzTPP]’代表对氯苄基三苯基锑阳离子,mnt^2-代表马来二氰基二硫烯阴离子）．配合物为三斜晶系,空间群Pi,晶胞参数为a=1．0585（2）nm,b=1．1089（2）nm,c=1．1570nm,α=81．98（1）°,β=84．95（1）°,γ=84．45（1）°,V=1．3691（4）nm^3,Z=1,最终一致性因子R=0．0584．该配合物由2个[ClBzT—PP]^＋阳离子,1个[M（mnt）2]^2-阴离子和1个H2O组成．其结构特点是配合物中的[C1BzTPP]’阳离子和Ni（mnt）2^2-阴离子沿c轴方向堆积成柱,并通过C—H…S,C—H…π,C—H…Ni氢键和π-π堆积作用形成了二维网状结构．     

Geometrical structure of benzene and naphthalene: ultrahigh-resolution laser spectroscopy and ab initio calculation

Geometrical structures of the isolated benzene and naphthalene molecules have been accurately determined by using ultrahigh-resolution laser spectroscopy and ab initio calculation in a complementary manner. The benzene molecule has been identified to be planar and hexagonal (D(6h)) and the structure has been determined with accuracies of 2 × 10(-14) m (0.2 mA?; 1 A? = 1 × 10(-10) m) for the C-C bond length and 1.0 × 10(-13) m (1.0 mA?) for the C-H bond length. The naphthalene molecule has been identified to be symmetric with respect to three coordinate axes (D(2h)) and the structure has been determined with comparable accuracies. We discuss the effect of vibrational averaging that is a consequence of zero-point motions on the uncertainty in determining the bond lengths.     

A Mixed‐Ligand Chiral Rhodium(II) Catalyst Enables the Enantioselective Total Synthesis of Piperarborenine B

A novel, mixed‐ligand chiral rhodium(II) catalyst, Rh₂(S‐NTTL)₃(dCPA), has enabled the first enantioselective total synthesis of the natural product piperarborenine B. A crystal structure of Rh₂(S‐NTTL)₃(dCPA) reveals a “chiral crown” conformation with a bulky dicyclohexylphenyl acetate ligand and three N‐naphthalimido groups oriented on the same face of the catalyst. The natural product was prepared on large scale using rhodium‐catalyzed bicyclobutanation/ copper‐catalyzed homoconjugate addition chemistry in the key step. The route proceeds in ten steps with an 8 % overall yield and 92 % ee.     

A 2D solid-state NMR experiment to resolve overlapping aromatic resonances of thiophene-based nematogens

In this study, we demonstrate the feasibility of resolving overlapping 13C chemical shift spectral lines of aromatic rings in a thiophene-based nematogen in the mesophase using a 2D PITANSEMA solid-state NMR method. This technique provided the information about chemical shift values as well as dipolar couplings that are used for determining the orientational order parameter. Large C-H dipolar coupling values measured for thiophene in contrast to phenyl rings suggest that the heterocyclic ring is not part of the molecular axis. Using the order parameter, we determined the orientation of C-H vectors of the thiophene ring. We believe that the 2D solid-state NMR can be extended to other types of liquid crystalline materials such as the banana-based mesogens for determining the orientational order and bent angle.     

Anomeric effects in the symmetrical and asymmetrical structures of triethylamine. Blue-shifts of the C-h stretching vibrations in complexed and protonated triethylamine

Quantum mechanical calculations using density functional theory with the hybrid B3LYP functional and the 6-31++G(d,p) basis set are performed on isolated triethylamine (TEA), its hydrogen-bond complex with phenol, and protonated TEA. The calculations include the optimized geometries and the results of a natural bond orbital (NBO) analysis (occupation of sigma* orbitals, hyperconjugative energies, and atomic charges). The harmonic frequencies of the C-H stretching vibrations of TEA are predicted at the same level of theory. Two stable structures are found for isolated TEA. In the most stable symmetrical structure (TEA-S), the three C-C bond lengths are equal and one of the C-H bond of each of the three CH2 groups is more elongated than the three other ones. In the asymmetrical structure (TEA-AS), one of the C-C bonds and two C-H bonds of two different CH2 groups are more elongated than the other ones. These structures result from the hyperconjugation of the N lone pair to the considered sigma*(C-H) orbitals (TEA-S) or to the sigma*(C-C) and sigma*(C-H) orbitals of the CH2 groups (TEA-AS). The formation of a OH...N hydrogen bond with phenol results in a decrease of the hyperconjugation, a contraction of the C-H bonds, and blue-shifts of 28-33 cm-1 (TEA-S) or 40-48 cm-1 (TEA-AS) of the nus(CH2) vibrations. The nu(CH3) vibrations are found to shift to a lesser extent. Cancellation of the lone pair reorganization in protonated TEA-S and TEA-AS results in large blue-shifts of the nu(CH2) vibrations, between 170 and 190 cm-1. Most importantly, in contrast with the blue-shifting hydrogen bonds involving C-H groups, the blue-shifts occurring at C-H groups not participating in hydrogen bond formation is mainly due to a reduction of the hyperconjugation and the resulting decrease in the occupation of the corresponding sigma*(C-H) orbitals. A linear correlation is established between the C-H distances and the occupation of the corresponding sigma*(C-H) orbitals in the CH2 groups.