HCN Dimer Research Articles

Electron and anion mobility in low density hydrogen cyanide gas. I. Dipole-bound electron ground states

We measured the mobility of excess electrons in the polar hydrogen cyanide gas (D=2.985 D) at low densities as a function of density and temperature by the so-called pulsed Townsend method. Experiments were performed at 294 and 333 K in the gas number density range 1.23×1017⩽n⩽3.61×1018 cm−3. We found a strong density dependence of the “zero-field” density-normalized mobility (μn). Only about 10% of the observed density variation can be qualitatively explained by coherent and incoherent multiple scattering effects. With increasing gas density an increasing number of linear HCN dimers is formed which due to the high dipole moment (D=6.552 D) represent much stronger electron scatterers than the HCN monomers. It was found that the dimers may be only in part responsible for the observed density effect. Therefore, we consider a transport process where short-lived dipole-bound electron ground states (lifetime ⩾12 ps) as quasilocalized states are involved. For comparison the electron mobility in saturated 2-aminoethanol vapor with a dipole moment of similar size (D=3.05 D) does not show any anomalous density behavior in the temperature range 298⩽T⩽435 K. In contrast to this the electron mobility in saturated but also in nonsaturated CH3CN gas (D=3.925 D) shows a density behavior similar to that in HCN.

Development of a potential function for describing the properties of HCN clusters

We developed potential functions for (HCN)2 and used them to study larger aggregates. The functions are based on ab initio perturbation calculations for the dimer and on molecular properties also estimated from ab initio calculations involving two different basis sets. The overall energy is resolved into electrostatic, repulsion, induction, and dispersion components. The electrostatic contribution is represented by a multipole expansion at several sites. The repulsion function is of the exponential type and includes atomic anisotropy. The dispersion is described by means of a London-type formula. Finally, the induction contribution is expressed in terms on polarizabilities distributed among the atoms. The proposed functions accurately reproduce the characteristics of the HCN dimer and the two minimum-energy forms of the trimer. Also, they predict various properties (e.g., the dependence of the dipole moment and interaction energy on cluster size) of larger aggregates.

A comparison of Hartree?Fock, MP2, and DFT results for the HCN dimer and crystal

A number of hydrogen-bond related quantities-geometries, interaction energies, dipole moments, dipole moment derivatives, and harmonic vibrational frequencies-were calculated at the Hartree-Fock, M ...

Rotational spectra and structures of the C6H6–HCN dimer and Ar3–HCN tetramer

A comparative study has been made of the rotational properties of C6H6–HCN and Ar3–HCN, observed with the Balle/Flygare pulsed beam, Fourier transform microwave spectrometer. C6H6–HCN is found to be a prolate symmetric top and Ar3–HCN an oblate one, both with the H in the middle. The rotational constants B0, DJ, and DJK of the parent species are 1219.9108(4) MHz, 1.12(3) kHz, and 18.32(8) kHz for C6H6–HCN, and 886.4878(1) MHz, 10.374(2) kHz, and 173.16(1) kHz for Ar3–HCN. Rotational constants are reported for the isotopic species C6H6–H13CN, -HC15N, and 13CC5H6–HC15N, and for Ar3–HC15N and -DCN. Analysis of the 14N hyperfine interaction χ finds its projection on the figure axis to be −4.223(4) MHz in C6H6–HCN and −1.143(2) in Ar3–HCN. They correspond to average projection angles θ between the HCN and figure axes of 15.2° and 45.3°, respectively. A pseudodiatomic analysis of the rotational constants gives the c.m. to c.m. distance to be 3.96 Å in C6H6–HCN and 3.47 Å in Ar3–HCN. While the rotational properties of C6H6–HCN are ‘‘normal,’’ those of Ar3–HCN display a long list of ‘‘abnormalities.’’ They include a J-dependent χ(14N) similar to that of Ar–HCN; a very large projection angle θ; large centrifugal distortion including higher-order terms in HJ and HJK; splitting of the K=3 transitions into J-dependent doublets; and the ready observation of an excited vibrational state. These behavioral differences are related qualitatively to the interaction surfaces for the two clusters, calculated with the molecular mechanics for clusters (MMC) model, and discussed. The potential minimum for C6H6–HCN is smooth, circular, steep except for a flat bottom, and deep (1762 cm−1). That for Ar3–HCN is tricuspid, with large gullies, and shallow (507 cm−1). In addition to the dispersion forces, the dominant interaction forming C6H6–HCN is between the benzene quadrupole moment and the HCN dipole moment, a strong 4-2 potential. That in Ar3–HCN is polarization of the spherical Ar by the HCN dipole and quadrupole moments, a weak 0–2,4 potential.

Excited ν3 vibrational state of the Ar–HCN and Kr–HCN dimers

Rotational spectra have been observed for an excited vibrational state of the Ar/Kr–HCN dimers in a Balle–Flygare Fourier transform microwave spectrometer with a high temperature nozzle that can be heated to ∼1300 K. The B, DJ, and HJ rotational constants found for the excited parent Ar–HCN isotopic species are 1602.475(3) MHz, 164.2(6) kHz, and 309(5) Hz, and for Kr–HCN they are 1176.4493(3) MHz, 41.7(1) kHz, and 56(1) Hz. The 14N quadrupole interaction χa(J=1) observed in the excited state is −2.824 (−3.239) MHz for Ar(Kr)–HCN, slightly smaller than the −2.844 (−3.268) MHz in the ground state. A substitution analysis based on the C and N isotopic species reveals that the C–N distance rs (CN) increases by ∼0.013 (0.020 Å) on excitation of Ar(Kr)–HCN. A similar analysis for free DC15N gives an increase in rs(CN) upon ν3 excitation of 0.0022 Å. Assignment of the excited state in the dimer as the ν3C–N stretch of the HCN is proposed and discussed.

Influence of polarization functions on atomic properties of bridge N and H atoms of HCN … HCN

The effect of different types of core and valence polarization functions on the equilibrium geometry, electric fields, Cioslowsky and Mulliken atomic charges, dissociation energy and CH stretching frequency changes for HCN … HCN was investigated. Basis sets for C, N and H were constructed (6-GCM) from the 6-31G basis sets to improve the Hellmann-Feynman forces and to describe adequately the asymptotic behavior of the wavefunction in the valence region. This new procedure provided accurate basis sets. Calculations of equilibrium geometry, total energies, harmonic frequencies and electric fields of some test molecules showed excellent agreement with accurate results. The resulting basis sets were considerably large in size and the need for reliable calculations of the hydrogen bonds of the HCN dimer with a 6-31G basis set using different procedures to include polarization functions was investigated. The properties calculated at the Hartree-Fock level of theory (SCF) for the HCN dimer using the 6-GCM basis sets were taken as reference. Three sets of polarization functions derived from the original 6-31G basis set were used with the original 6-31G basis set at the SCF level of theory for the HCN dimer. The agreement between the properties calculated with the polarized 6-31G basis set and the standard 6-GCM basis was analyzed using the simple statistical Euclidean distance. Comparison of the Euclidean distances for molecular properties (electric field, Cioslowsky charges, bond lengths, change in CH stretching frequency and dissociation energies) suggests that 6-31G polarized with the Pople's original d type functions or mixing Pople's polarization function with polarization that improves the Hellmann-Feynman forces are in closer agreement with the calculations using the 6-GCM basis set. If molecular properties more directly related to the hydrogen bond are improved, other molecular properties are also better characterized. Euclidean distances for Mulliken charges have shown a different tendency. If only Mulliken charges are considered, the polarization of hydrogen is sufficient.

Rotational spectrum and structure of an (H2O–HCN)–Ar trimer

Rotational transitions of the (H2O–HCN)–Ar trimer have been measured at 3–17 GHz with the Balle/Flygare Mark II pulsed nozzle FT microwave spectrometer for the parent, 18O, 13C, 15N, D2O, and HDO isotopic species. The isotopomers exhibit both a- and b-dipole transitions with 14N hyperfine structure and all but the HDO have two sets of transitions assigned to 000 and 101 internal rotational states of the H2O or D2O. Rotational constants were determined by fitting the line centers separately for each isotopic set to the Watson Hamiltonian for an asymmetric top. A molecular mechanics for clusters (MMC) calculation of the potential energy surface and an approximate substitution analysis of the rotational constants give a nearly planar, Δ-shaped structure which is a somewhat distorted superposition of the H2O–HCN, H2O–Ar, and Ar–HCN dimers. MMC also gives a barrier of ≲25 cm−1 to internal rotation of the H2O. Factors governing the formation of trimers are discussed. The effects on trimer structure of differences in the pair interactions are found to be appreciable while the role of three-body effects is small.

Rotational spectrum and structure of the Ne–HCN dimer

Microwave rotational transitions have been observed at low J (0–3) for several isotopic species of the Ne–HCN dimer using the Balle/Flygare Mark II Fourier transform spectrometer with a pulsed nozzle as the source. For 20Ne–HC 14N, the main K=0 transitions give rotational constants B̄, DJ, and HJ of 2772.816 and 1.280 MHz and 1.173 kHz. The 14N nuclear quadrupole constant increases linearly with J(J+1) at a slope Dχ of −12.7 kHz from a value for χa(14N) of −0.957 MHz at J=0. The pseudodiatomic approximation for B̄ and χa(14N) leads to a value of 3.89 Å for the Ne to HC 14N center-of-mass (c.m.) distance R, and to 46.8° for the ‘‘average’’ bend angle θ of HC 14N. Some of the K=0, J=1→2, and J=2→3 transitions exhibit one or two weak satellites ∼30 MHz away, usually below, but also both above and below. The J=1→2 low frequency satellites for 20Ne–HC 14N and 20Ne–HC 15N, nominally 111→212, are symmetrical doublets with splittings of 305 and 439 kHz, respectively. The 14N hyperfine structure (hfs) is identical for the two 20Ne–HC 14N components as is the Stark effect for 20Ne–HC 15N. The molecular mechanics for clusters (MMC) model was used to calculate potential energy surfaces for Rg–HCN dimers, giving stabilities of 21, 37, 85, and 108 cm−1 with He, Ne, Ar, and Kr as the rare gas. A qualitative comparison of the experimental properties for the dimers with Ne, Ar, and Kr as the rare gas is based on the surfaces. The extremely mobile internal dynamics of Ne–HCN are attributed to its potential surface, which is both very shallow and isotropic.

Structure and dynamics of the H2O–HCN dimer

This report extends an earlier microwave study of the H2O –HCN weakly bonded dimer by Legon [Proc. R. Soc. London, Ser. A 396, 405 (1984)]. We have resolved the H–H(H2O) hyperfine structure (hfs) in rotational transitions of H2O–HC15N and the 17O hfs in H217O–HC15N, using a modified Balle/Flygare Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the sample source. Also, the rotational constants of H2O–H13CN have been determined. The hfs, particularly that of 17O, and a substitution analysis, are used to clarify the dynamics of the dimer. The analyses support a pseudoplanar, H2O –HCN, C2v structure in which the H2O and HCN experience in-plane and out-of-plane bending vibrations of modest on average amplitude. The out-of-plane H2O bend is 20° and the in-plane is perhaps half that. The bending of the HCN is isotropic, with an amplitude of 9.4° in both directions. The molecular mechanics for clusters (MMC) model was used to explore the potential energy surface (PES) for the weak-bonding coordinates. The calculated equilibrium structure differs greatly from the experimental, with the H2O rotated out of plane by 60° in one direction and the HCN by 20° in the other (cis). The difference is shown by the 17O hfs and its dependence upon the H2O bending to be caused by the zero-point vibrational averaging of the structure, which extends over the shallow symmetric double minimum in the PES. The interaction energy is large (−1590 cm−1 ), but the PES is relatively flat in the bending coordinates over large regions between the equilibrium minima, making the vibrational averages differ substantially from the equilibrium values.

J dependence of χa(14N) and χa(83Kr) for the Kr–HCN dimer

High-resolution microwave rotational spectra for 84Kr–HCN, 86Kr–HCN, and 83Kr–HC15N have been observed with the pulsed-nozzle, Fourier transform Balle/Flygare Mark II spectrometer. A new method of injecting the gas sample into the Fabry–Perot cavity along the axis of the microwave pulse was used for some transitions to narrow the linewidths. The present work extends that of the original study [J. Chem. Phys. 78, 3483 (1983)] over a wider frequency range (2–18 GHz) and with higher resolution. The 14N nuclear quadrupole coupling constant has been found to increase linearly with J(J+1) for 84Kr–HCN and 86Kr–HCN, with the slope Dχ one-third its value for the analogous Ar–HCN dimer. For 84Kr–HCN, the average HCN bending amplitude θ decreases from 26.85° for J=0 to 26.28° for J=7, while the average Kr to HCN center-of-mass (c.m.) distance R increases from 4.5202 to 4.5246 Å. Similar results are found for 86Kr–HCN. In addition, the 83Kr quadrupole coupling constant for 83Kr–HC15N is dependent on J, increasing from 7.5382 MHz for J=1 to 7.5713 MHz for J=4. This is interpreted with the long-range polarization model used previously to explain rare gas nuclear quadrupole coupling constants in Rg–HX dimers. In particular, the J dependence observed for χa(83Kr) is consistent with the J dependencies of θ and R inferred from χa(14N) in the 14N species. Radial and angular motions of HCN are strongly coupled.

HCN dimers: iminoacetonitrile and N-cyanomethanimine

Iminoacetonitrile (1) has been prepared by two methods: (i) thermal decomposition of the tosylhydrazone salts 13 at 200-degrees-C and (ii) Ar matrix photolysis of azidoacetonitrile (15). Ab initio calculations indicate that 1Z is of slightly lower energy than 1E, and this is confirmed by the IR spectra with use of the thermal methods. 1E/1Z undergo photochemical interconversion, giving a ca. 3:1 photostationary E:Z ratio. 1E and 1Z are fully characterized by their gas-phase, matrix, and thin-film IR spectra, which are in excellent agreement with ab initio calculations, by H-1 and C-13 NMR spectroscopy in solution, and by mass spectrometry. 1 polymerizes in solution above -40-degrees-C; pyrolysis produces HCN, and matrix photolysis produces HNC and van der Waals complexes containing HNC. N-tert-Butyliminoacetonitrile thermally fragments to tert-butyl isocyanide and HCN. N-Cyanomethanimine (3) has also been prepared by two methods: (i) pyrolysis of trimethylenetetrazole (20) at 500-800-degrees-C and (ii) pyrolysis of ditetrazolopyrazine 22 at 600-850-degrees-C. Both methods are extremely clean. 3 is fully characterized by its IR spectrum in agreement with ab initio calculations and, in conjunction with other work, by its mass and millimeter-wave spectra. 3 is thermodynamically stable in the gas phase up to ca. 800-degrees-C at low pressure and short contact times but polymerizes in the solid state above -100-degrees-C.

Mutual polarization of monomer charge distribution in (HCN)2, (HCN)3, and (HCN)∞

A simple classical mutual polarization model accurately predicts the induced dipole moments of linear HCN dimer and trimer. The model employs the first four nonzero electrical molecular moments of each HCN, and both the molecular polarizability and the C–H and C–N bond dipole polarizabilities. The model is extended to linear H-bonded oligomers up to the pentamer, and also to an infinite linear H-bonded chain, appropriate for comparison with the H-bonded chains present in the HCN crystal. For the dimer and trimer, a relation is seen between the change in the electric field gradient at the N of HCN and the calculated induced C–N bond dipole moment. A semiquantitative proportionality constant, derived based on this relation, is used to enhance understanding of the underlying cause of the large increase in the 14N quadrupole coupling constant in the HCN crystal relative to the free HCN monomer. This increase results from the strong polarization of the C–N bond by the local field at each HCN caused by the rest of the HCN crystal lattice.

Rotational spectra and structures of small clusters containing the HCN dimer: X–(HCN)2 with X=CO, N2, NH3, and H2O

This work is the counterpart of a previous report on the (HCN)2–Y trimers with Y=HF, HCl, HCF3, and CO2 [J. Chem. Phys. 90, 4069 (1989)]. Rotational spectra have been observed for several isotopic species of the OC–, N2–, H3N–, and H2O–(HCN)2 trimers, using a pulsed nozzle, Fourier transform Balle/Flygare microwave spectrometer. The structures are basically composites of those reported for the (HCN)2 and X–HCN dimers. The trimers are effectively axially symmetric, but have some shrinkage of dimensions. Rotational constants found for the main isotopic species of each trimer are: For X=OC, a B0 of 421.142 MHz and DJ of 110 Hz; for X=N2, 435.573 MHz and 155 Hz; for X=H3N, a symmetric top, a B0 of 675.777 MHz, DJ of 180 Hz, and DJK of 41.1 kHz; and for X=H2O, with C2v symmetry, a (B0+C0)/2 of 667.028 MHz, (B0−C0)/2 of 0.617 MHz, DJ of 173 Hz, and a DJK of 62.9 kHz. The rotational constants for the isotopic species of each trimer were used to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers, r1 being that for X–HCN and r2 that for (HCN)2. For X=OC, N2, H3N, and H2O the shrinkages found in r1 are 0.068, 0.056, 0.084, and 0.074 Å, respectively, and in r2 0.013, 0.013, 0.044, and 0.026 Å. The 14N quadrupole coupling constants were determined by selective 15N substitution for most of the nitrogen sites in the trimers. The effects of charge redistribution in the trimers were separated from those of torsional oscillations in several instances including N2 in N2–(HCN)2.

Rotational spectrum and potential surface for Ar2–HCN: A T-shaped cluster with internal rotation

The rotational spectrum of Ar2–HCN has been observed between 2.5 and 11.5 GHz with the pulsed nozzle, Fourier transform, Balle/Flygare Mark II microwave spectrometer. Eighteen transitions were found and their 14N quadrupole hyperfine structure analyzed. The line centers were fitted with the Watson Hamiltonian giving ground state rotational constants of 1769.366, 1743.854, and 857.600 MHz. The effective geometry of the cluster is found to be T-shaped with C2v symmetry and the H end of the HCN closest to the Ar2. However, the rms deviation of the fit is poor (300 kHz), the centrifugal distortion and inertial defect are huge, the Ar to HCN c.m. distance is nearly 0.2 Å shorter than in the Ar–HCN dimer, and the average angular displacement of the HCN from the C2 axis is both large (39°) and highly anisotropic (10°). In contrast, the Ar2 subunit exhibits an in-plane, average angular displacement of only 6°. These anomalies led us to calculate potential surfaces for Ar2–HCN and Ar2–HF using the molecular mechanics for clusters scheme. A comparison of the surfaces and the rotational properties of the two species prompts us to propose that in Ar2–HCN the HCN axis rotates about the C2 axis maintaining an angle of ∼40° between them for the m=0 internal rotation state. Such internal rotation accounts at least qualitatively for the otherwise anomalous rotational behavior of the Ar2–HCN cluster.

ChemInform Abstract: Microwave Spectra, Dipole Moments, and Energy Difference of E and Z C-Cyanomethanimine (HCN Dimer)

Abstract Microwave spectra of a hydrogen cyanide dimer, C-cyanomethanimine (HNCHCN), and its partially deuterated (DNCHCN) species produced by the pyrolysis of dimethylcyanamide were observed and analyzed. The rotational constants determined are (in MHz): A B C E HNCHCN 62 700.34(21) 4972.056(5) 4600.303(5) Z HNCHCN 54 156(48) 5073.849(38) 4632.425(38) E DNCHCN 62 718.55(35) 4682.488(12) 4351.655(11) Z DNCHCN 46 023(66) 4988.459(42) 4493.772(39) The energy difference between the E and Z conformers was determined to be 215(50) cm −1 from measurements of the relative intensity and dipole moments for the normal species. The possible mechanism of the pyrolytic reaction for the formation of C-cyanomethanimine from dimethylcyanamide is discussed.

Microwave spectra, dipole moments, and energy difference of E and Z C-cyanomethanimine (HCN dimer)

Abstract Microwave spectra of a hydrogen cyanide dimer, C-cyanomethanimine (HNCHCN), and its partially deuterated (DNCHCN) species produced by the pyrolysis of dimethylcyanamide were observed and analyzed. The rotational constants determined are (in MHz): A B C E HNCHCN 62 700.34(21) 4972.056(5) 4600.303(5) Z HNCHCN 54 156(48) 5073.849(38) 4632.425(38) E DNCHCN 62 718.55(35) 4682.488(12) 4351.655(11) Z DNCHCN 46 023(66) 4988.459(42) 4493.772(39) The energy difference between the E and Z conformers was determined to be 215(50) cm −1 from measurements of the relative intensity and dipole moments for the normal species. The possible mechanism of the pyrolytic reaction for the formation of C-cyanomethanimine from dimethylcyanamide is discussed.

Energy hypersurface for HCN dimer: Problems in choosing the configuration space R

The energy hypersurface for the HCN·HCN dimer was analyzed and the topography investigated. The presence of various local minima was found. The importance in defining the configuration space R is stressed, and the connection between the number of stationary points and the choice of R discussed.

Potential energy surfaces for the HCN dimer and trimer hydrogen-bonded complexes

Ab-initio SCF-MO potential-energy surfaces for the HCN dimer and trimer have been calculated using the STO/4-31G basis set. The equilibrium geometries, energies and one-electron properties were also calculated. Charge-redistribution effects are discussed in terms of Mulliken population analysis indices. The low-energy path connecting the minima on the potential-energy surfaces for the transformation between the different structures is also discussed.

Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Y with Y=HF, HCl, HCF3, and CO2

Microwave rotational spectra have been observed for a number of isotopic species of the (HCN)2–HF, –HCl, –HCF3, and –CO2 trimers. The observations were made with the pulsed nozzle, Fourier transform, Flygare/Balle Mark II spectrometer. The trimers have structures which are composites of the linear (HCN)2 dimer and the HCN–Y dimers, the latter linear for Y=HF and HCl, a symmetric top for Y=HCF3, and T-shaped with C2v symmetry for Y=CO2. The rotational constants for the most abundant species of each trimer are as follows: For Y=HF and HCl, B0 is 699.204 and 467.408 MHz, respectively, and DJ is 162 and 87 Hz; for Y=HCF3, B0 is 305.742 MHz and DJ and DJK are 51 and 471 Hz; for Y=CO2, treated as a symmetric top, (B0+C0)/2 is 452.426 MHz and DJ is 1.057 kHz. Hyperfine interaction constants were determined for several species. The B0’s for each trimer were analyzed by a combination of isotopic substitution and fitting procedures to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers. The B0’s are relatively insensitive to the position of the central HCN but give r1+r2 accurately. With this limitation, r1 and r2 in the trimers are compared with the corresponding distances in the dimers, which are longer. For Y=HF, HCl, HCF3, and CO2, respectively, the shrinkages found in r1 are 0.069, 0.054, 0.030, and 0.004 Å and in r2, 0.043, 0.062, 0.042, and 0.052 Å. The shrinkage in r1 and several other properties of the trimers exhibit some correlation with the pseudodiatomic stretching force constant in the HCN–Y dimer.

Sub-Doppler rotationally resolved overtone spectroscopy of the HCN dimer

A new molecular beam color center laser spectrometer, which can be tuned single mode under computer control around 1.5 μm, has been used to obtain the sub-Doppler, rotationally resolved, first overtone spectrum of the nonbonded C–H stretch vibration of the HCN dimer. As for the corresponding fundamental vibration, the linewidth is instrument limited, posing a minimum limit of 11 ns to the lifetime of the excited complex.