Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for 2-chloroethyl silane

The infrared (3500–30 cm⁻¹) spectra of gaseous and solid and the Raman (3500–10 cm⁻¹) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH₂CH₂SiH₃, have been recorded. Similar data have been recorded for the Si–d₃ isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−55 to −100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm⁻¹ (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si–H frequencies from the Si–d₂ isotopomer the r_o Si–H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.     

Conformational and structural studies of ethyl fluorosilane from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

The infrared spectra (3200–30 cm⁻¹) of gaseous and solid ethyl fluorosilane, CH₃CH₂SiH₂F, have been recorded. Additionally, the Raman spectra (3200–30 cm⁻¹) of the liquid and solid have been recorded and quantitative depolarization values obtained. Both the gauche and trans conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 54±16 cm⁻¹ (646±191 J/mol) with the gauche conformer the more stable form. This is consistent with the predictions from ab initio, MP2/6-311+G(2d,2p), calculation as well as those with smaller basis sets with full electron correlations. A complete vibrational assignment is proposed for both the trans and gauche conformers based on infrared band contours, relative intensities, depolarization values, and group frequencies, which are supported by normal-coordinate calculations utilizing the force constants from MP2/6-31G(d) ab initio calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311+G(2d,2p) at levels of restricted Hartree–Fock (RHF) and/or Moller Plesset to the second order (MP2) with full electron correlation. The adjusted r₀ parameters have been obtained for both conformers from a combination of the previously reported rotational constants with ab initio predicted values. All results are compared to similar quantities of some corresponding molecules.     

Infrared and Raman spectra, conformational stability, ab initio calculations of structure, and vibrational assignment of 2-hexyne

The infrared spectra (3500–50 cm⁻¹) of the gas and solid and the Raman spectra (3500–50 cm⁻¹) of the liquid and solid have been recorded for 2-hexyne, CH₃–CC–CH₂CH₂CH₃. Variable temperature studies of the infrared spectrum (3500–400 cm⁻¹) of 2-hexyne dissolved in liquid krypton have also been recorded. Utilizing four anti/gauche conformer pairs, the anti(trans) conformer is found to be the lower energy form with an enthalpy difference of 74±8 cm⁻¹ (0.88±0.10 kJ/mol) determined from krypton solutions over the temperature range −105 to −150 °C. At room temperature it is estimated that there is 42% of the anti conformer present. Equilibrium geometries and energies of the two conformers have been determined by ab initio (HF and MP2) and hybrid DFT (B3LYP) methods using a number of basis sets. Only the HF and DFT methods predict the anti conformer as the more stable form as found experimentally. A vibrational assignment is proposed based on the force constants, relative intensities, depolarization ratios from the ab initio and DFT calculations and on rotational band contours obtained using the calculated equilibrium geometries. From calculated energies it is shown that the CH₃ group exhibits almost completely free rotation which is in agreement with the observation of sub-band structure for the degenerate methyl vibrations from which values of the Coriolis coupling constants, ζ, have been determined. The results are compared to similar properties of some corresponding molecules.     

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignments for methylaminothiophosphoryl difluoride

Infrared spectra (4000–50 cm⁻¹) of the vapor, amorphous and crystalline solids and Raman spectra (3600–10 cm⁻¹) of the liquid with qualitative depolarization data as well as the amorphous and crystalline solids of methylaminothiophosphoryl difluoride, CH₃N(H)P(=S)F₂, and three deuterated species, CD₃N(H)P(=S)F₂, CH₃N(D)P(=S)F₂, and CD₃N(D)P(=S)F₂, have been recorded. The spectra indicate that in the vapor, liquid and amorphous solid a small amount of a second conformer is present, whereas only one conformer remains in the low temperature crystalline phase. The near-infrared spectra of the vapor confirms the existence of two conformers in the gas phase. Asymmetric top contour simulation of the vapor shows that the trans conformer is the predominant vapor phase conformer. From a temperature study of the Raman spectrum of the liquid the enthalpy difference between the trans and near-cis conformers was determined to be 368±15 cm⁻¹ (4.41±0.2 kJ/mol), with the trans conformer being thermodynamically preferred. Ab Initio calculations with structure optimization using the 6-31G(d) and 6-311+G(d,p) basis sets at the restricted Hartree–Fock (RHF) and/or with full electron correlation by the perturbation method to second order (MP2) support the occurrence of near-trans (5° from trans) and near-cis (20° from cis) conformers. From the RHF/6-31G(d) calculation the near-trans conformer is predicted to be the more stable form by 451 cm⁻¹ (5.35 kJ/mol) and from the MP2/6-311+G(d,p) calculation by 387 cm⁻¹ (4.63 kJ/mol). All of the normal modes of the near-trans rotamer have been assigned based on infrared band contours, depolarization values and group frequencies and the assignment is supported by the normal coordinate calculation utilizing harmonic force constants from the MP2/6-31G(d) ab initio calculations.     

Vibrational spectra and density functional study of propylgermane

Raman and infrared spectra of propylgermane, CH₃CH₂CH₂GeH₃, and its Ge-deuterated analog, CH₃CH₂CH₂GeD₃, were investigated in their gaseous, liquid and solid states. The normal coordinate treatment was carried out by density functional theory (DFT) calculation, using B3LYP/6-31G^* and 6-311++G^** basis sets, and the corresponding fundamental vibrations were assigned. The trans (T) and gauche (G) forms around the central C–C bond coexisted in the gaseous and liquid states and only the T form existed in the solid state. From the temperature dependent measurements of the Raman spectra in the liquid state, the enthalpy difference was found to be ΔH(T−G)=−0.36±0.02 kcalmol⁻¹ with the T form being more stable. The energy differences between the isomers obtained by DFT calculations were ΔE(T−G)=−0.46 kcalmol⁻¹ and ΔE(T−G)=−0.87 kcalmol⁻¹ by the 6-31G^* basis set and 6-311++G^** basis set, respectively.     

Rotational isomerism and conformational energies in 2-haloethanols

The Raman spectra of 2-fluoroethanol have been obtained as a function of temperature in the liquid and in the solid phases. The results of these experiments have provided substantial evidence for the existence of trans conformers in the liquid phase. The above study, coupled with analysis of the temperature dependence of the C-X stretching vibrations of the other haloethanols in the pure liquid, has permitted, for the first time, a determination of ΔH for all four members of the series. An interpretation is offered to explain the observed trends in the conformational energy differences.     

Conformational studies of cyclopropylmethyl isocyanate from temperature dependent FT-IR spectra of xenon solutions and ab initio calculations

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3200 to 100 cm⁻¹) of cyclopropylmethyl isocyanate, c-C₃H₅CH₂NCO, dissolved in liquefied xenon, have been carried out. The infrared spectra (gas and solid) as well as the Raman spectrum of the liquid have been recorded from 3200 to 100 cm⁻¹. By analyzing six conformer pairs in xenon solutions, an enthalpy difference of 193 ± 19 cm⁻¹ (2.31 ± 0.23 kJ/mol) was obtained with the gauche–cis rotamer (the first designation indicates the orientation of the CNCO group with respect to the three-membered ring, the second designation indicates the relative orientation of the NCO group with respect to the bridging CC bond) the more stable form and the only form present in polycrystalline solid. The abundance of the cis–trans conformer present at ambient temperature is 16 ± 1%. The potential function governing the conformational interchange has been obtained from B3LYP/6-31G(d) calculations and the two-dimensional potential has been obtained. From MP2 ab initio calculations utilizing various basis sets with diffuse functions, the gauche–cis conformer is predicted to be more stable by 223 to 269 cm⁻¹, which is consistent with the experimental results. However, without diffuse functions the predicted conformational energy differences are much smaller (77–166 cm⁻¹). Similar diffuse function dependency affects density functional theory calculations by the B3LYP method to a lesser extent. A complete vibrational assignment for the gauche–cis conformer is proposed and several fundamentals for the cis–trans conformer have been identified. The structural parameters, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and r₀ structural parameters are estimated. These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

Vibrational spectra, conformations, ab initio calculations and vibrational assignments of 3-pentyn-2-ol

The infrared spectra of 3-pentyn-2-ol, CH₃CCCH(OH)CH₃, have been recorded as a vapour and liquid at ambient temperature, as a solid at 78 K in the 4000–50 cm⁻¹ range and isolated in an argon matrix at ca. 5 K. Infrared spectra of the solid phase at 78 K were obtained before and after annealing to temperatures of 120 and 130 K. The IR spectra of the solid were quite similar to that of the liquid.

Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, some bands changed in relative intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer in which OH is oriented anti to C₁(a_Me). From various bands assigned to a second conformer in which OH is oriented anti to H_gem(a_H), the conformational enthalpy differences was found to be between 0.4 and 0.8 kJ mol⁻¹. The highest energy conformer with OH anti to C₃(a_C) was not detected.

Quantum-chemical calculations have been carried out at the MP2 and B3LYP levels with a variety of basis sets. Except for small basis set calculations for which the a_H conformer had slightly lower energy, all the calculations revealed that a_Me was the low energy conformer. The B3LYP/cc-pVTZ calculations suggested the a_Me conformer as more stable by 0.8 and 8.3 kJ mol⁻¹ relative to a_H an a_C, respectively. Vibrational wavenumbers and infrared and Raman band intensities for two of the three conformers are reported from B3LYP/cc-pVTZ calculations.     

X-ray diffraction studies on cis- and trans-4'-cyanobiphenyl-4-yl 3-n-propylcyclobutane-1-carboxylate

X-ray diffraction has been used to study the nematic phases of the cis and trans isomers of an elongated molecule containing the cyclobutane group. It has been shown that these nematic phases consist of a mixture of single molecules and overlapping core dimers. It has been found that increasing the temperature of the cis isomer rapidly reduces the dimer population, whereas increasing the temperature of the trans isomer enhances the dimer population. This provides an explanation of the relatively high clearing temperature of the trans isomer. The different temperature dependences of the dimer population in the cis and trans isomers has been explained by a model involving inversion of the cylobutane group.     

Conformational and structural studies of 1-chloropropane and 1-bromopropane from temperature-dependant FT-IR spectra of rare gas solutions and ab initio calculations

Variable temperature (−55 to −150°C) studies of the infrared spectra (3500–400 cm⁻¹) of 1-chloropropane (CH₃CH₂CH₂Cl) and 1-bromopropane (CH₃CH₂CH₂Br) dissolved in liquid krypton and xenon, respectively, have been recorded. Utilizing two conformer pairs in krypton solution for chloride and three conformer pairs in xenon solution for bromide, enthalpy differences of 52±3 cm⁻¹ (0.62±0.06 kJ/mol) and 72±7 cm⁻¹ (0.86±0.08 kJ/mol) were obtained for the chloride and bromide, respectively, with the gauche form being the more stable conformer for both molecules. From these data, it is estimated that 28 and 26% of trans form are present at ambient temperature for the chloride and bromide, respectively. The conformation stabilities, harmonic force constants, fundamental frequencies, infrared intensities and Raman activities have been obtained from RHF/6-31G(d) and/or MP2/6-31G(d) ab initio calculations for both halopropanes and these quantities have been compared to the experimental values when appropriate. The optimized geometries have also been obtained with several different ab initio basis sets with full electron correlation by the perturbation method up to MP2/6-311+G(2d,2p). The r₀ structural parameters of both halopropanes have been obtained by combining the ab initio data with the previously reported microwave rotational constants for both conformers. The quantities are compared to the corresponding results for some similar molecules.     

Trans-cis isomerization of an azoxybenzene liquid crystal

Trans-cis isomerization was investigated in a room temperature liquid crystal mixture of two azoxybenzene compounds. Experiments were performed on isolated molecules in dilute solutions and on the liquid crystal phase composed of the pure compounds. The absorption spectra of the trans and cis isomers were found to be similar to those of azobenzene compounds, as were the birefringence and order parameter of the nematic liquid crystal phase. The photo-optic properties were also similar in that irradiation by ultraviolet light caused the conversion from trans to cis isomers, while short wavelength visible light incident on these compounds resulted in the conversion from cis to trans isomers. The activation energy for thermal relaxation from the cis to trans isomer in the liquid crystal phase was determined to be (66±7) kJ/mole, which is less than for azobenzene in solution. While a photostationary state in a dilute solution with approximately equal numbers of trans and cis isomers was achieved, the nematic-isotropic transition of the mixture of the pure compounds decreased from 70°C to room temperature with a cis concentration of only about 12%. One unusual finding was that the photostationary concentration of trans and cis isomers due to irradiation with light of a specific visible wavelength depended on the starting concentrations of the two isomers, indicating that there may be a molecular conformation that is not photo-responsive and relaxes only thermally.     

On the vibrational spectra and conformational stability of 1,1-dimethylhydrazine from temperature dependent FT-IR spectra of krypton solutions and ab initio calculations

Variable temperature (−105 to −150 °C) studies of the infrared spectra (3500–400 cm⁻¹) of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm⁻¹ less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r₀ parameters from MP2/6-311+G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm⁻¹ with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm⁻¹ and that of the outer one of 1662 cm⁻¹ from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852±16 and 1558±12 cm⁻¹, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V′₃₃ (sin 3τ₀ sin 3τ₁ term) value which ranged from 190 to 232 cm⁻¹ is in reasonable agreement with the experimental value of 268±3 cm⁻¹ but the predicted V₃₃ (cos 3τ₀ cos 3τ₁ term) value of −73 to −139 cm⁻¹ is 25% smaller and of the opposite sign of the experimental value of 333±22 cm⁻¹. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules.     

Conformational stability of 1-bromo-2-fluoroethane from temperature dependent FT-IR spectra of rare gas solutions

Variable temperature (−55 to −135°C) studies of the infrared spectra (3500–400 cm⁻¹) of 1-bromo-2-fluoroethane, BrCH₂CH₂F, dissolved in liquid krypton and xenon have been recorded. From these data, the enthalpy difference has been determined to be 108±9 cm⁻¹ (1.296±0.113 kJ/mol) and 112±8 cm⁻¹ (1.346±0.098 kJ/mol) from the krypton and xenon solutions, respectively, with the trans conformer the more stable rotamer. Complete vibrational assignments are presented for both conformers which are consistent with the predicted frequencies obtained from the ab initio MP2/6-31G^* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been obtained from RHF/6-31G^* and/or MP2/6-31G^* ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate. Structural parameters and conformational stability have also been obtained from MP2/6-311+G^** calculations. Combining the ab initio predicted structural parameters with the microwave rotational constants, r_o parameters have been obtained for the gauche conformer.     

Acide glyoxylique Symétrie de la molécule libre, spectres de vibration et attribution

The infrared and Raman spectra of glyoxylic acid in the gas phase, in solution and in the solid state have been obtained in the region from 4000 to 300 cm⁻¹. p]Band structure analysis in the gaseous state shows that the configuration of formyl-carbonic acid is trans and planar. It was established that in solution some bands belong to dioxyacetic acid. p]A vibrational assignment is proposed based on a priori calculation of the spectra, using a Urey-Bradley force field intermediate between those of glyoxal and oxalic acid.     

The manifestation of non-bonded attraction in the physical properties of cis and trans olefins

Non-bonded attraction is suggested to account for a host of differences in the physical properties of cis and trans olefins of the type XHC=CHX. The main predictions are: (i) The cis isomer is more stable than the trans isomer; (ii) The C=C bond is longer and the C-X bonds are shorter for the cis isomer; (iii) The π MO's orbital energies of the two isomers differ such that the trans isomer is a better electron donor and electron acceptor than the cis isomer. Ab initio calculations at the STO-3G and the 4-31G levels in support of the model are presented. The photoelectron spectra of cis and trans difluoro, dichloro and dibromoethylene are discussed, and found to be in accord with our qualitative model.     

Rotational isomerism about acyl–oxygen bond in two envelope conformations and vibrational spectral analysis of cyclopentyl acetate—a combined theoretical and experimental study

The s-cis and s-trans isomers resulting from the rotation about the acyl–oxygen bond of two envelope conformations with C5 (neighbour to substituted carbon C4) and C4 as apical atoms in the five-membered ring and vibrational spectra of cyclopentyl acetate are studied with density functional molecular orbital theory at the B3LYP/6-311++G** level. In the case of C5 at the flap and –OAc group in the axial position, it is found that the s-cis isomer (1:s-cis) is more stable than the s-trans isomer (1:s-trans) by 7.46 kcal/mol. The s-cis–s-trans rotational barrier is 15 kcal/mol. The other two conformers with C4 at the flap and –OAc group in the equatorial position, the relative energies of the s-cis and s-trans isomers (2:s-cis and 2:s-trans) with respect to 1:s-cis are found to be 0.45 and 8.21 kcal/mol, respectively. The infrared spectra (200–3200 cm⁻¹) in gas and liquid phase and Raman spectra (3200–150 cm⁻¹) in liquid phase for cyclopentyl acetate and 10 of its isotopomers are recorded. The calculated spectra of all conformers along with the observed spectra has helped study the effect of rotational isomerism on the vibrational spectra. The normal coordinate analysis in terms of non-redundant local coordinates is done for vibrational assignments of the 57 normal modes. The experimental and theoretical results are compared to the corresponding quantities of some similar molecules.     

Infrared, and Raman spectra, conformational stability, normal coordinate analysis, ab initio calculations, and vibrational assignment of 1-chlorosilacyclobutane

The infrared spectra (3500–40 cm⁻¹) of gaseous and solid and the Raman spectra (3500–30 cm⁻¹) of liquid and solid 1-chlorosilacyclobutane, c-C₃H₆SiClH, have been obtained. Both the axial and equatorial conformers with respect to the chlorine atom have been identified in the fluid phases. Variable temperature (−105 to −150°C) studies of the infrared spectra of the sample dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 211±17 cm⁻¹ (2.53±0.21 kJ/mol), with the equatorial conformer being the more stable form and the only conformer remaining in the annealed solid. At ambient temperatures, approximately 26% of the axial conformers are present in the vapor phase. A complete vibrational assignment is proposed for the equatorial conformer, and many of the fundamentals of the axial conformers have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, and depolarization ratios have been determined for both rotamers by ab initio calculations employing the 6-31G(d) basis set at the levels of restricted Hartree–Fock (RHF) and/or Moller–Plesset (MP) to second order. Structural parameters have also been obtained using MP2/6-311+G(d,p) ab initio calculations. The r₀ parameters for both conformers are obtained from a combination of the ab initio predicted values and the twelve previously reported microwave rotational constants. The results are discussed and compared to those obtained for some similar molecules.     

Rotational isomerism in 1,4-disilabutane

Raman spectra of 1,4-disilabutane (DSB) are recorded in both liquid and solid states. IR spectra in the gas and solid phases are also recorded. Two rotational isomers, trans and gauche, are present in the gas and pure liquid phases. An assignment of the main features of the IR and Raman spectra is made, with the aid of normal coordinate calculations for the two isomers. The temperature dependence of the Raman spectrum of DSB is studied in the range 77–300 K. The enthalpy difference between the two isomers is 5.01 ± 0.31 kJ mol^?1. This value is expected to be higher for gaseous DSB.     

Raman,infrared and NMR spectra and structure of divinylmethylborane

The Rarnan spectra of gaseous, liquid and solid divinylmethylborane have been recorded from 20–3500 cm^?1 and the IR spectra of gaseous and solid divinylmethylborane recorded over the range 30–3500 cm^?1. A variable temperature study of the Raman spectrum of the liquid phase has been carried out. A complete vibrational assignment is presented. In the solid phase the molecule appears to have a planar heavy atom skeleton (C_s symmetry). From analysis of the spectra of the fluid phases, the presence of a second isomer, in which one or both of the vinyl groups are twisted slightly out of the BC₃ plane (C₁ symmetry), is proposed. Variable temperature ¹³C NMR studies have been carried out. A comparison of the ¹³C chemical shift of C_β of the vinyl group with the corresponding value in other vinylboranes indicates that relatively little delocalization of the π-electron density occurs in this molecule. Low temperature (?115°C) ¹³C NMR data are consistent with a low barrier to rotation about the boron-vinyl carbon bond.     

Infrared matrix isolation and quantum chemical studies of the nitrous acid complexes with water

The 1:1 and 2:1 complexes between water and trans- and cis-isomers of nitrous acid have been isolated in argon matrices and studied using FTIR spectroscopy and DFT(B3LYP) calculations with a 6-311++G(2d,2p) basis set. The analysis of the experimental spectra indicate that 1:1 complexes trapped in solid argon involve very strong hydrogen bond in which acid acts as the proton donor and water as the proton acceptor. The perturbed OH stretches are −248, −228 cm⁻¹ red shifted from their free-molecules values in complexes formed by trans- and cis-HONO isomers, respectively. The calculated spectral parameters for the two complexes are in good agreement with experimental data. The calculations also predict stability of two more 1:1 weakly bound complexes formed by each isomer. In these the water acts as the proton donor and one of the two oxygen atoms of the acid as the acceptor. The experimental spectra demonstrate also formation of 2:1 complex between water and trans-HONO isomer in an argon matrix. The performed calculations indicate that the complex involves a seven-membered ring in which OH group of HONO forms very strong hydrogen bond with the oxygen atom of one water molecule and nitrogen atom acts as a weak proton acceptor for the hydrogen atom of the second water molecule of the water dimer. The observed perturbations of the OH stretch of trans-HONO (750 cm⁻¹ red shift) is much larger than that predicted by calculations (556 cm⁻¹ red shift); this difference is attributed to strong solvation effect of argon matrix on very strong hydrogen bond.