D.J. Millen

Systematic effect of d substitution on hydrogen-bond lengths in gas-phase dimers B…HX and a model for its interpretation

The hydrogen-bond lengths r(B…H-X) and r(B…D-X) have been redetermined on a consistent basis for the axially symmetric dimers B…HX, where B=N2, CO, PH3, HCN, CH3CN, NH3, or (CH3)3N and X=F, Cl or CN. For the series B…HF and B…HCl, the difference in the effective zero-point bond lengths δr=r(B…H-X)-r(B…D-X) is found to decrease as the hydrogen bond strength (kσ) increases. This behaviour is interpreted in terms of the effect of D substitution on the opposing contributions to the zero-point motion of the two modes (νS and νB) that involve large-amplitude displacements of the H atom.

Microwave spectrum and nonplanarity of cyanamide

Abstract Microwave transitions of the isotopic species H2NCN, HDNCN, and D2NCN have been obtained and assigned. A number of vibrational satellites are observed in each case. The origin of these is considered, and it is concluded that the equilibrium configuration about the amino nitrogen is pyramidal and that there is a low-lying molecular “inversion” level. A vibration-rotation perturbation in the case of D2NCN indicates that for this molecule the level is about 15 cm−1 above the ground state.

Observation of c-type transitions and H, F nuclear spin—nuclear spin coupling in the rotational spectrum of (CH2)2O…HF

Abstract Ground-state rotational transitions allowed by the component μ c of the electric dipole moment in the dimer (CH 2 ) 2 O…HF have been measured and lead to improved rotational constants. The absence of inversion doubling in these transitions allows a lower limit to be placed on the barrier to inversion of the configuration at oxygen. The magnitude of the determined H, F spin—spin coupling constant suggests that the O…H—F system may be non-linear.

Vibrational spectra and vibrational states of simple gas-phase hydrogen-bonded dimers

Abstract The article aims to outline the growth of evidence and ideas about infrared band broadening for simple, gaseous, moderately strong, hydrogen bonded dimers B…HA, to draw attention to areas in need of further development and to collect together experimental information available at the present time about vibrational states associated with such dimers. The band associated with the modified HA stretching mode, ν s , is observed for several dimers not only to be broad, but to have sub-band structure, which is satisfactorily interpreted as arising from combination bands of ν s with the low frequency stretch-mode ν σ , giving a progression ν s ± nν σ as a result of strong anharmonic coupling. For weaker dimers with lower values of ν σ the sub-band structure is less evident, and may appear only as shoulders, while for still weaker dimers, the sub-bands may be merged into a featureless broad band. A major factor contributing to the breadth of the individual sub-bands in the band structure is the presence of hot-bands, especially a long series based on successive excited states of the low-frequency bending mode ν β . A link is indicated with the interpretation of band-broadening for moderately strong hydrogen-bonded complexes in the liquid state. The anharmonic coupling of ν s and ν σ is again a central feature but there is a new factor, namely the coupling of ν σ through a fluctuating potential with the surroundings, which has the result that the ν s mode rapidly loses phase coherence resulting in a broad structureless band.

Experimental determination of the dissociation energies D0 and De of H2O…HF

Abstract The zero-point and equilibrium dissociation energies of the hydrogen-bonded dimer H 2 O…HF have been determined as D 0 = 34.3(3) kJmol −1 and D e = 42.9 (8) kJmol −1 from absolute intensities of rotational transitions in an equilibrium mixture of H 2 O, HF and H 2 O…HF.

Electric-Charge Redistribution in the HCl Subunit of Hydrogen-Bonded Dimers B

An interpretation is given of the chlorine nuclear quadrupole coupling constants X(Cl) for the series of dimers B • • • HCl and B • • • DCl where B = CO, C2H4, C2H2, PH3, H2S, HCN, CH3CN, H2O and NH3. The factors that contribute to the change in X(Cl) on dimer formation are considered in turn. First, account is taken of the effect of bond lengthening of the HCl subunit that occurs on dimer formation. Secondly, the contribution XE to the change in the coupling constant that arises from the electrical effect of B on the field gradient at the Cl nucleus in the dimer is treated at equilibrium in terms of two contributions according to the equation XE = XP + XQ = ‒eQ{Fzz Fz + Gzz Fzz}/h. The first term XP results from the polarization of the HCl subunit by the electric field Fz due to B. The second term XQ arises from the field gradient Fzz due to B but modified by the factor (l + γzz) = Gzz, where γzz is the usual Sternheimer antishielding factor. Fzz is the corresponding factor associated with the field gradient at the Cl nucleus resulting from the polarization of the HCl subunit by the field due to B. The term XQ is directly evaluated using an available Sternheimer antishielding factor. Thirdly, allowance is made for the effect of averaging over the zero-point bending motion of the dimer. Finally, the remaining term XP has then been calculated for each member of the series B • • • HC1 and shown to be linearly dependent on Fz as required by the above expression. Hence it has been possible for the first time to make an experimental determination of an Fzz value of a gas-phase molecule and we report Fzz = ‒116(6) x 1010 m-1 for the HCl molecule.

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The rotational spectrum of the hydrogen cyanide dimer has been observed in the frequency range 26-40 GHz by using a Stark-modulated microwave spectrometer. Although the spectrum is very weak, even under optimum conditions, it has been possible to assign vibrational satellites in the vβ progression based on the ground state and in the vβ progression based on vσ ═ 1 with the aid of the computer simulation of spectra and the ground-state spectroscopic constants. The spectroscopic constants now available for the hydrogen cyanide dimerare summarized as follows: (HC14N)2 (DC14N)2 (HC15N)2 vβ ═ 1 ← 0/cm-1 35±5 30±5 35±5 vσ ═ 1 ← 0/cm-1 101 ─ ─ Bo/MHz 1745.80973(50) 1661.18(26) 1684.28825(25) DJ/kHz 2.133(30) (1.873) 1.900(30) rc. m./nm 44.496 ─ 44.499 Kσ/(Nm-1) 8.14 ─ 8.51 αβ/MHz ─20.07 (2) ─17.73 (27) ─18.74 (9) γβ/MHz 0.266 (4) 0.242 (36) 0.250 (17) qβ/MHz 5.33 (4) 5.44 (13) 5.15 (10) ασ/MHz (31.44) ─ ─

HCl from Cl Nuclear Quadrupole Coupling: Determination of the Coefficient

The rotational spectra of six isotopic species of HCN∙ ∙ ∙HF in their vibrational ground states have been observed by pulsed-nozzle, Fourier-transform, microwave spectroscopy and have been analysed to yield the rotational constants B0, centrifugal distortion constants DJ and various hyperfine coupling constants Xi(D), X3(14N) and D4, 5 as follows (subscripts refer to the numbering scheme H(1)C(2)N(3)∙ ∙ ∙H(4)F(5)). isotopic species B0/ MHZ DJ/kHZ Xi(D)/kHZ X3(14N)/MHZ D4, 5(H, F)/kHZ HC15N∙ ∙ ∙HF 3573 5874(2) 6.97(2) ─ ─ –242(6) HC15N∙ ∙ ∙DF 3551.5110 6.83 269(4)(i = 4) ─ ─ HC14N∙ ∙ ∙HF 3591.1552 6.99 ─ –4.098(4) –218(6) HC14N∙ ∙ ∙DF 3569.6576 6.86 259(2)(i = 4) –4.096(1) ─ DC15N∙ ∙ ∙HF 3360.3549 5.85 186(5)(i = 1) ─ –244(4) DC15N∙ ∙ ∙DF 3338.0824 5.81 { 181(10) (i = 1) ─ ─ 283(7) (i = 4) The B0 values lead to conclusions about the dimer geometry, the DJ values allow the hydrogen bond stretching force constant kσ to be determined and the hyperfine coupling constants provide information about the internal dynamics of the subunits. An analysis of the D nuclear quadrupole coupling and H, F nuclear spin nuclear-spin coupling constants demonstrates that the H—F bond lengthens by 0.14 Å when the heterodimer HCN∙ ∙ ∙HF is formed. (1 Å = 10–10 m = 10–1 nm.)

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Abstract A method is described for determination of the dissociation energy D0 for hydrogen bonded dimers B…H-A using only measurements of rotational transition intensities at a single temperature. Application in the particular case HCN…HF gives D0 = 18.5 ± 1.1 kJ mol−1. By taking account of the vibrational modes of HCN…HF in the harmonic oscillator approximation, De is estimated as 25.6 ± 1.6 kJ mol−1.

of the Electric-Field-Induced Field Gradient in HCl

We report a procedure that for the first time allows the quantitative determination of the lengthening δr of the HF bond on formation of an isolated hydrogen-bonded dimer B···HF. We show how δr can be obtained from the ground-state hyperfine coupling constants X (D nuclear quadrupole) and DHF (H, F nuclear spin to nuclear spin), which are available from the rotational spectra of the molecules B···H(D)F. We establish the values of δr for the series of heterodimers B···HF where B = Ar, Kr, Xe, N2, CO, H2S, HCN, CH3CN and H2O. The values of δr are zero for the first three members of this series and then 0.001, 0.007, 0.010, 0.014, 0.016 and 0.015 Å, respectively (1 Å = 10-1 nm = 10-10 m). It is found that δr is a monotonically increasing function of the hydrogen-bond stretching force constant kσ, which is taken as a convenient measure of the hydrogen bond strength.