H₃P⋯AgI: generation by laser-ablation and characterization by rotational spectroscopy and ab initio calculations

Abstract

The new compound H₃P⋯AgI has been synthesized in the gas phase by means of the reaction of laser-ablated silver metal with a pulse of gas consisting of a dilute mixture of ICF₃ and PH₃ in argon. Ground-state rotational spectra were detected and assigned for the two isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI in their natural abundance by means of a chirped-pulse, Fourier-transform, microwave spectrometer. Both isotopologues exhibit rotational spectra of the symmetric-top type, analysis of which led to accurate values of the rotational constant B₀, the quartic centrifugal distortion constants D_J and D_JK, and the iodine nuclear quadrupole coupling constant χ_aa(I) = eQq_aa. Ab initio calculations at the explicitly-correlated level of theory CCSD(T)(F12*)/aug-cc-pVDZ confirmed that the atoms P⋯Ag–I lie on the C₃ axis in that order. The experimental rotational constants were interpreted to give the bond lengths r₀(P⋯Ag) = 2.3488(20) Å and r₀(Ag–I) = 2.5483(1) Å, in good agreement with the equilibrium lengths of 2.3387 Å and 2.5537 Å, respectively, obtained in the ab initio calculations. Measures of the strength of the interaction of PH₃ and AgI (the dissociation energy D_e for the process H₃P⋯AgI = H₃P + AgI and the intermolecular stretching force constant F_P⋯Ag) are presented and are interpreted to show that the order of binding strength is H₃P⋯HI < H₃P⋯ICl < H₃P⋯AgI for these metal-bonded molecules and their halogen-bonded and hydrogen-bonded analogues.

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1 Introduction

A programme of systematic investigations of small molecules of the type B⋯MX is being conducted, where B is a small Lewis base (e.g. N₂, OC, H₂O, H₂S, HC≡CH, H₂C=CH₂, cyclopropane or NH₃), M = Cu, Ag or Au, and X = F, Cl or I.^1–19 The programme has both experimental and theoretical components. The experimental approach is to produce B⋯MX by laser ablation of the metal M in the presence of a gas pulse composed of small amounts of B and a molecular source of halogen atoms X in a large excess of argon. Following supersonic expansion of the product B⋯MX entrained in the carrier gas, its rotational spectrum is observed in isolation at a low effective temperature. Various properties of B⋯MX are available through analysis of the rotational spectrum, namely the angular geometry, the distances r(B⋯M) and r(M–X), the strength of the intermolecular bond B⋯M, and the electric charge redistribution that accompanies formation of B⋯MX. The theoretical component of the investigations involves ab initio calculations at the CCSD(T)(F12*) explicitly correlated level of theory, usually with the largest basis set affordable. These calculations have the advantage of providing accurate properties of the isolated molecule, which can be compared with the experimental results.

Several molecules H₃N⋯MX, where M = Cu or Ag and X = F, Cl or I, have been detected and characterised recently in the gas phase for the first time through their rotational spectra,^17–19 although H₃N⋯CuCl was identified in the solid state earlier.²⁰ Each was established to be a symmetric-top molecule, with the N⋯MX nuclei lying on the top (C₃) axis, in the order indicated. To date, analogues of H₃N⋯MX having phosphine instead of ammonia as the Lewis base B have not been identified experimentally, to the best of our knowledge, but several have been the subject of density functional calculations.^21,22 We report here the rotational spectrum of H₃P⋯Ag–I and some of its properties derived therefrom.

There is some evidence that molecules B⋯MX (M = Cu, Ag, or Au; X = F, Cl, or I)^1–19 have geometries that are isomorphic with those of their hydrogen-bonded (B⋯HX, X is a halogen atom)²³ and halogen-bonded (B⋯XY, XY is a dihalogen molecule)²⁴ counterparts, but are more strongly bound and exhibit a greater electric charge rearrangement within the diatomic subunit. Our interest here is to examine the geometry and binding strength of H₃P⋯Ag–I and the electric charge redistribution within Ag–I that accompanies its formation. These properties will then be compared with those of the closely related molecule H₃N⋯CuI,¹⁹ with those of their hydrogen-bonded analogues H₃P⋯HI²⁵ and H₃N⋯HI²⁶ and with those of their halogen-bonded relatives, H₃P⋯ICl²⁷ and H₃N⋯ICl.²⁸

2 Experimental and theoretical methods

2.1 Detection of the rotational spectrum

A chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer fitted with a laser ablation source was used to observe rotational spectra in the frequency range 6.5 to 18.5 GHz. Detailed descriptions of the spectrometer and laser ablation source are available elsewhere.^29,30 A gas sample containing ∼4.0% PH₃ and ∼1.5% CF₃I in argon was prepared at a total pressure of 6 bar. The sample was pulsed over the surface of a silver rod that was ablated by a suitably timed Nd:YAG laser pulse (wavelength 532 nm, pulse duration 10 ns, pulse energy 20 mJ). Subsequently, the gas pulse expanded supersonically into the vacuum chamber of the spectrometer. The rod was translated and rotated regularly at small intervals to allow each laser pulse (repetition rate of ∼1.05 Hz) to impinge on a fresh metal surface and thereby ensure shot-to-shot reproducibility.

The sequence employed to record broadband microwave spectra involves repetition of two steps. The first is polarization of the sample by a microwave chirp that sweeps from 6.5 to 18.5 GHz within 1 μs and the second is recording of the subsequent free induction decay of the molecular emission over a 20 μs time period. This sequence is repeated eight times during the expansion of each gas sample pulse into the spectrometer chamber. The free induction decay (FID) of the polarization is mixed down with the signal from a 19 GHz local oscillator and then digitized by means of a 25 Gs s⁻¹ digital oscilloscope. Each transition is observed as a single peak with full-width at half-maximum (fwhm) ≅ 150 kHz after application of a Kaiser–Bessel digital filter.

2.2 Ab initio calculations

Structure optimizations and counter-poise corrected dissociation energies were calculated using the Turbomole package³¹ at the CCSD(T)(F12*) level of theory,³² a coupled-cluster method with single and double excitations, explicit correlation,³³ and a perturbative treatment of triple excitations.³⁴ Only valence electrons were included in the correlation treatment. A basis set combination consisting of aug-cc-pVDZ on H and P atoms and aug-cc-pVDZ-PP on Ag and I atoms was used and will be referred to by AVDZ. ECP-10-MDF^35,36 and ECP-28-MDF³⁷ were used on Ag and I, respectively, to account for scalar relativistic effects. For the density fitting approximation used to accelerate the CCSD(T)(F12*) calculation, the respective def2-QZVPP basis sets were employed for the MP2^38,39 and Fock⁴⁰ terms. For the complementary auxiliary basis required for the F12 treatment,⁴¹ the aug-cc-pCVDZ MP2 density fitting basis sets were used.³⁹ Quadratic force constants were also calculated at this level of theory. For comparison, the same force constants were calculated with the GAUSSIAN 09 package⁴² at the MP2 level of theory. A basis set combination consisting of aug-cc-pVTZ on the H and P atoms, and aug-cc-pVTZ-PP on the Ag and I atoms was used in this case.

3 Results

3.1 Determination of spectroscopic constants

The observed spectrum of H₃P⋯AgI showed evidence of the presence of the two isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI, each exhibiting iodine nuclear quadrupole hyperfine structure, as may be seen from consideration of Fig. 1. An iterative least-squares fit of the observed hyperfine frequencies of each isotopologue was conducted using the program PGOPHER, written and maintained by Western.⁴³ The Hamiltonian employed was of the formwhere H_R is the usual energy operator appropriate to a semi-rigid symmetric rotor molecule and −⅙Q:∇E is the iodine nuclear quadrupole energy operator, in which Q is the iodine nuclear electric quadrupole moment tensor and ∇E is the electric field gradient tensor at I. The matrix of H was constructed in the coupled symmetric-rotor basis I + J = F. The only determinable spectroscopic constants were the rotational constant B₀, the quartic centrifugal distortion constants D_J and D_JK, and the iodine nuclear quadrupole coupling constant χ_aa(I) = −eQ∂²V/∂a² = eQq_aa (where q_aa = −∂²V/∂a² is the electric field gradient along the C^a₃ axis direction). The magnetic coupling of the iodine nuclear spin to the molecular rotation can in principle be described by the spin-rotation constant C_bb but this constant was too small to determine from the observed frequencies. Values of the spectroscopic constants from the final cycle of the least-squares fit with PGOPHER are given in Table 1 for the two isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI investigated, together with σ_RMS, the RMS deviation of the fit, and N, the number of hyperfine components fitted. Spectra simulated using PGOPHER and the final set of spectroscopic constants are shown in Fig. 1. The detailed PGOPHER fits are available as Supplementary Material. The values of σ_RMS are satisfactory, given the estimated accuracy of frequency measurement (12 kHz) associated with the chirped-pulse F-T microwave spectrometer.

Fig. 1 Top panel: (a) broadband spectrum recorded while probing a sample containing CF₃I, Ag and PH₃ (530k FIDs). Some transitions of CF₃I are taken off-scale to allow weaker transitions to be distinguished from the baseline. (b) Expanded section of spectrum displayed in (a) to show J′ − J′′ = 5–4 transitions of ¹⁰⁷AgI (∼13 420 MHz) and ¹⁰⁹AgI (∼13 280 MHz). Hyperfine splittings in each transition are evident. (c) Expanded section of the spectrum displayed in (a) to show J′ − J′′ = 11–10 transition of H₃P⋯¹⁰⁹AgI (black). A simulated spectrum that uses the results of fitted parameters is displayed inverted (red).

Table 1 Observed spectroscopic constants^a of H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI

Spectroscopic constant	H3P⋯^107AgI	H3P⋯^109AgI
a Numbers in parentheses are one standard deviation in units of the last significant digits. b Standard deviation of the fit. c Number of hyperfine frequencies included in the fit.
B 0/MHz	626.01307(23)	624.76423(17)
D J /kHz	0.03182(89)	0.03238(64)
D JK /kHz	4.46(14)	4.04(10)
χ aa (I)/MHz	−733.83(34)	−734.54(27)
σ RMS /kHz	12.0	9.0
N	88	93

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3.2 Molecular geometry

The facts that the ground-state rotational spectrum of the detected complex of phosphine and argentous iodide is of the symmetric-top type and that the Ag atom is close to the complex centre of mass (see later) mean that the arrangement of the atoms is either H₃P⋯AgI or PH₃⋯AgI. The second of these is unlikely because ^δ+Ag–I^δ− is dipolar in the indicated sense and it is expected that the positive end of the electric dipole would interact with the P non-bonding electron pair, which lies on the C₃ axis of phosphine. This expectation is confirmed by ab initio calculations at the CCSD(T)(F12*)/AVDZ level of theory, which predict that the optimised geometry of PH₃⋯AgI lies higher in energy by 116 kJ mol⁻¹ than that of the H₃P⋯AgI conformer. The higher energy conformer would not be populated at the low effective temperature (∼2 K) of the supersonic expansion. The observed conformer is therefore of the general form shown in Fig. 2.

Fig. 2 The molecular geometry of H₃P⋯AgI drawn to scale. The internal coordinates r₁ and r₂ used in the discussion of how to obtain force constant F₂₂ from the centrifugal distortion constants D⁰_J are indicated. The experimental zero-point values of r₁ and r₂ are r₀(Ag–I) = 2.5483(1) Å and r₀(P⋯Ag) = 2.3488(20) Å, respectively.

The rotational constants B₀ = C₀ for the two isotopologues H₃³¹P⋯¹⁰⁷Ag¹²⁷I and H₃³¹P⋯¹⁰⁹Ag¹²⁷I allow only a partial determination of the lengths of the H–P, P⋯Ag and Ag–I bonds and of the angle α = ∠HPAg (between the P–H bond and the C₃ axis) necessary to define the r₀ geometry. The quantities of most interest are r₀(P⋯Ag) and r₀(Ag–I). The ab initio calculations indicate that r_e(P–H) decreases by 0.0114 Å when phosphine enters the complex and the angle α_e decreases by 3.94°. We shall assume that the r₀ geometry of phosphine (r₀(P–H) = 1.420003 Å and angle α₀ = 122.86° obtained by fitting the accurately known⁴⁴B₀ and C₀ using the STRFIT program of Kisiel⁴⁵) changes in the same way as does the r_e geometry on formation of H₃P⋯Ag–I. If so, r₀(P–H) = 1.4086 Å and α₀ = 118.92° are appropriate to PH₃ in the complex. When these values were assumed in a fit of the ground-state principal moments of inertia of H₃³¹P⋯¹⁰⁷Ag¹²⁷I and H₃³¹P⋯¹⁰⁹Ag¹²⁷I, the values r₀(P⋯Ag) = 2.3488 Å and r₀(Ag–I) = 2.5483 Å resulted. No errors in these quantities are generated in the fit because two constants are fitted by two parameters. However, calculations reveal the following variations: ∂r(P⋯Ag)/∂r(P−H) = 0.065, ∂r(Ag−I)/∂r(P−H) = 0.005, ∂r(P⋯Ag)/∂α = 0.002 Å deg⁻¹. and ∂r(Ag−I)/∂α = 0.0001 Å deg⁻¹. Thus, the length r₀(Ag–I) is very insensitive to changes to the geometry of PH₃ that might occur when H₃P⋯AgI is formed. These partial derivatives lead, when the reasonable errors of δr₀ = 0.005 Å and δα₀ = 1° are assumed, to r₀(P⋯Ag) = 2.3488(20) Å and r₀(Ag–I) = 2.5483(1) Å. The results from the CCSD(T)(F12*)/AVDZ optimisation of H₃P⋯AgI are 2.3387 Å and 2.5537 Å, respectively.

The fact that spectroscopic constants have been determined for the isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI allows the coordinate a_Ag to be obtained by the substitution method from the expression

a_Ag² = ΔI⁰_b/μ_s, (2)

in which ΔI⁰_b is the difference in the zero-point moments of inertia of the two isotopologues and where M is the mass of the parent and Δm is the mass change accompanying the isotopic substitution at Ag. The result is |a_Ag| = 0.9017(17) Å, where the error is estimated from δa = 0.0015/|a| as recommended by Costain.^46,47 The corresponding values for this coordinate implied by the determined r₀ geometry and the ab initio r_e geometry are 0.9017 Å and 0.9056 Å, respectively.

3.3 Strength of the interaction of H₃P and AgI

There are two common measures of the strength of the interaction of phosphine and silver iodide in H₃P⋯AgI. Both are properties of the one-dimensional potential-energy function associated with variation of the distance r(P⋯Ag) when C_3v symmetry is maintained but with structural relaxation at each point (referred to as the dissociation coordinate). The first is the intermolecular stretching quadratic force constant F_P⋯Ag. The second is the energy, D_e, required to dissociate H₃P⋯AgI to give PH₃ and AgI at infinite separation, with reactants and products at their equilibrium geometries. The first can be obtained from the experimental centrifugal distortion constants D_J but the second is not available from the present experiments. Both are available from the ab initio calculations.

For weakly bound complexes (such as most hydrogen-bonded complexes B⋯HX, where B is a simple Lewis base and X is a halogen atom) it is a good approximation to assume that B and HX are rigid and unchanged in geometry on complex formation. Then F_B⋯H can be related to the equilibrium centrifugal distortion constant D^e_J or Δ^e_J (depending on molecular symmetry) of the complex and the various rotational constants of B, HX and B⋯HX, as demonstrated by Novick⁴⁸ for the case where B is an atom and by Millen⁴⁹ for a wider range of molecules B. For complexes B⋯MX, where M is a coinage metal atom, the intermolecular bond can be strong and the approximation that the force constant F_B⋯M is much smaller than all other stretching force constants is no longer appropriate. To deal with such cases, we have recently described a two-force constant model which relates the quadratic force constants F_M–X and F_B⋯M (hereafter referred to as F₁₁ and F₂₂, respectively) to either D^e_J or Δ^e_J under the assumption that the contribution of the cross term F₁₂ is negligible.⁵⁰ The model applies to all complexes of a Lewis base B with any diatomic molecule (e.g. a hydrogen halide HX, a dihalogen XY, or a coinage metal halide MX) as long as the diatomic molecule lies along a C_n (n ≥ 2) symmetry axis of B in the equilibrium geometry. Note that B is assumed rigid, but can be changed in geometry when subsumed into the complex. During the vibrational motion no further change is assumed, however.

The two-force constant model for a symmetric-top molecule such as H₃P⋯AgI leads (with numbering of the Ag and I atoms and internal coordinates r₁ and r₂ shown in Fig. 2) to the expression⁵⁰

In eqn (3), I^e_bb is an equilibrium principal moment of inertia and the a_n are equilibrium principal axis coordinates of atoms n = 1 and 2. The compliance matrix elements (F⁻¹)_nn are simply 1/F_nn under the approximations described above. It was shown in ref. 50 that zero-point constants and coordinates can be used in place of equilibrium values to a reasonable approximation. Least-squares fitting of (F⁻¹)₁₁ and (F⁻¹)₂₂ simultaneously to the D⁰_J values of the two isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI led to ill-conditioning, however, so instead a fixed value of F₁₁ was assumed and F₂₂ was fitted. Fig. 3 shows F₂₂ plotted as a function of F₁₁ for a wide range of values of the latter, with the equilibrium value of the force constant 145.8 N m⁻¹ of the free diatomic molecule Ag–I indicated, as calculated from its equilibrium vibrational wavenumber.⁵¹ If it is assumed that F₁₁ is unchanged from the equilibrium value in free AgI of 145.8 N m⁻¹, the result is F₂₂ = 122(5) N m⁻¹, where the error is that transmitted from the fit of the D⁰_J values.

Fig. 3 Values of the quadratic intermolecular stretching force constant F₂₂ obtained by fitting the centrifugal distortion constants D⁰_J of the isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI using eqn (3) at fixed values of the AgI stretching force constant F₁₁ in the range 130 to 220 N m⁻¹. Eqn (3) is valid only if the off-diagonal force constant F₁₂ is assumed to be zero. The CCSD(T)(F12*)/AVDZ-F12 value of F₁₁, the experimental equilibrium value for the free AgI molecule and the F₂₂ values they correspond to are indicated.

It is also possible to calculate F₁₁ and F₂₂ab initio. At the CCSD(T)(F12*)/AVDZ level of theory the results are F₁₁ = 151.1 N m⁻¹ and F₂₂ = 106.8 N m⁻¹. When the D⁰_J values are fitted by using eqn (3) with F₁₁ fixed at 151.1 N m⁻¹, the result is F₂₂ = 110(5) N m⁻¹, where the error is that implied by the error in the D⁰_J values, and is the best present experimental estimate for this quantity. For free AgI at the same level of theory, F₁₁ = 145.9 N m⁻¹ is obtained, in excellent agreement with the experimental equilibrium value of 145.8 N m⁻¹. Thus. F₁₁ increases by 3.5% when AgI is incorporated into H₃P⋯AgI. For comparison, the lower level of theory MP2/aug-cc-pVTZ-PP gives F₁₁ = 168.8 N m⁻¹ and F₂₂ = 130.1 N m⁻¹ when using the GAUSSIAN package.⁴² The result for free AgI at the same level is F₁₁ = 160.1 N m⁻¹, corresponding to 9.8% overestimation of the experimental equilibrium value. If F₁₁ for H₃P⋯AgI were also overestimated by a similar percentage, the corrected value would be F₁₁ = 153 N m⁻¹, which likewise represents a small increase relative to that of the free molecule.

It has been shown that in the limit of rigid, unchanged B and MX geometries, when F₁₁ becomes infinite, eqn (3) reduces to the corresponding Millen expression⁴⁹

in which B_B⋯MX, B_B and B_MX are equilibrium rotational constants of the complex and its components, but zero-point values are used of necessity. In eqn (4), μ = m_Bm_MX/(m_B + m_MX).

When B = H₃P and MX = AgI, a fit of the centrifugal distortion constants D⁰_J of H₃P⋯¹⁰⁷AgI and H₃P ¹⁰⁹AgI (using zero-point rotational constants given in Tables 1 and 2) leads to F₂₂ = 31.3(5) N m⁻¹, which is a very serious underestimate. The reason why becomes clear when the plot of F₂₂ as a function of F₁₁ is extended to cover a wider range of F₁₁ values and unphysical solutions for which F₂₂ is negative are included. The result is the rectangular hyperbola shown in Fig. 4. The horizontal asymptote (F₁₁ = ∞) gives F₂₂ = 31.26 N m⁻¹ and corresponds to the solution when AgI is rigid and unperturbed when within H₃P⋯AgI. The vertical asymptote (108.39 N m⁻¹) corresponds to the lowest possible value of F₁₁ consistent with the observed D⁰_J. Clearly, any reasonable F₁₁ must lead to a F₂₂ value that is considerably greater than that given by eqn (4).

Table 2 Some properties of H₃P and Ag–I

Property	H3P^a	Property	^107AgI^b	^109AgI^b
a Ref. 44. b Ref. 55. c Calculated by fitting the zero-point rotational constants using the program STRFIT (ref. 45). d Calculated from the equilibrium vibrational wavenumber ωe given in ref. 51 by using the expression FAgI = 4π^2ωe^2c^2μAgI, where μAgI = mAgmI/(mAg + mI).
B 0/MHz	133480.1165(17)	B 0/MHz	1342.99237(7)	1329.61831(7)
C 0/MHz	117489.4357(77)	χ aa (I)/MHz	−1062.5299(15)	−1062.5230(14)
r 0(P−H)/Å	1.42000^c	r 0(Ag−I)/Å	2.546627	2.546617
∠(HPH)/°	93.345^c	F AgI/(N m^−1)	145.78(3)^d	145.76(3)^d

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Fig. 4 The rectangular hyperbola obtained by following the procedure described in the caption to Fig. 3, but with the range of assumed F₁₁ values extended from −100 to +300 N m⁻¹. The negative values of F₂₂ and F₁₁ are unphysical. The asymptote at F₁₁ = 108.39 N m⁻¹ represents the value of that force constant below which a negative, unphysical value of F₂₂ is required to fit the centrifugal distortion constants D⁰_J. The asymptote at F₂₂ = 31.26 N m⁻¹ is the value of this force constant in the limit F₁₁ = ∞ N m⁻¹, that is when the MX molecule is rigid. It can be shown⁵⁰ that if both PH₃ and AgI were rigid and unperturbed on formation of H₃P⋯AgI eqn (3) leads to the Millen eqn (4), when equilibrium spectroscopic constants are used in the latter.

The other measure of the strength of binding is the dissociation energy defined earlier; it takes the value D_e = 116 kJ mol⁻¹ when calculated at the CCSD(T)(F12*)/AVDZ level of theory, after counterpoise correction.⁵² The value for AgI = Ag + I at the same level of theory is 230 kJ mol⁻¹. It is therefore clear from the D_e value and the force constant F₂₂ that the intermolecular bond in H₃P⋯AgI is by no means weak. In fact by either measure, the P⋯Ag bond is about an order of magnitude stronger than most hydrogen or halogen bonds, but is only about a factor of two weaker than the Ag–I bond itself.

3.4 Electric charge redistribution on formation of H₃P⋯AgI

The iodine nuclear quadrupole coupling constant χ_aa(I) = eq^I_aaQ^I carries information about the electric charge distribution at I through the electric field gradient q^I_aa along the a-axis direction at the iodine nucleus. According to the Townes–Dailey model⁵³ for interpreting such coupling constants, the ionicity i_c (or fractional ionic character) of the free AgI molecule is given byin which q^I_(5,1,0) is the contribution to the electric field gradient at I along the a-axis direction that arises from an electron in 5p_a orbital. The quantity eQ^Iq^I_(5,1,0) has the value 2292.71 MHz when described as a frequency.⁵⁴Eqn (5) leads to the result i_c = 0.537 for ¹⁰⁷Ag¹²⁷I (the values^51,55 of several properties of AgI, including χ_aa(I), are collected in Table 2) but has the value 0.680 for the complex H₃P⋯¹⁰⁷Ag¹²⁷I. Evidently, the charge rearrangement within AgI on formation of the complex is significant, a result consistent with the similar magnitude of the values for the dissociation energies D_e for the processes H₃P⋯AgI = H₃P + AgI and AgI = Ag + I referred to earlier. Interestingly, there appears to be very little change in its bond length when AgI is subsumed into the complex.

4 Conclusions

The new molecule H₃P⋯Ag–I has been synthesized in the gas phase by a laser ablation method in which a pulse of gas mixture consisting of a few per cent each of PH₃ and ICF₃, with the remainder Ar, interacts with the plasma produced when silver is ablated by a Nd-YAG laser operating at 532 nm. The product was detected and characterised by means of its rotational spectrum, as observed with a chirped-pulse, Fourier-transform microwave spectrometer. The molecule is a symmetric top of C_3v symmetry with the atoms P, Ag and I lying in that order on the symmetry axis a. Spectroscopic constants determined by fitting the observed transitions of the isotopologues H₃P⋯¹⁰⁷AgI and H₃P⋯¹⁰⁹AgI were interpreted to give the values r₀(P⋯Ag) = 2.3488(20) Å and r₀(Ag–I) = 2.5483(1) Å for the indicated bond lengths, after assuming changes to the r₀ geometry of free PH₃ when bound up in the complex were the same as the corresponding changes in the r_e geometry, as predicted ab initio at the CCSD(T)(F12*)/AVDZ level of theory. It is of interest to note that the value of r₀(Ag–I) is increased by only 0.0017 Å relative to the free AgI value of 2.54663 Å (see Table 2).⁵⁵ Interpretation of the centrifugal distortion constants D⁰_J and the iodine nuclear quadrupole coupling constants led to a value F₂₂ = 110 (5) N m⁻¹ for the quadratic stretching force constant of the P⋯Ag bond and to a value δi_c = 0.14 for the increase in the Ag–I bond ionicity when H₃P⋯AgI is formed. Although the ionicity of AgI increases significantly when subsumed into the complex, we note that the length of the bond and its force constant F₁₁ are effectively unchanged.

H₃N⋯CuI, synthesized and characterised recently by a similar method,¹⁹ is isomorphic with H₃P⋯AgI and has r₀(N⋯Cu) = 1.9357(13) Å and r₀(Cu–I) = 2.3553(5) Å, the latter representing an increase of only 0.0147 Å relative to the free Cu–I value of 2.34059 Å. The N⋯Cu interaction strength, as measured by F₂₂ = 110(30) N m⁻¹, is similar to that 110(5) N m⁻¹ of P⋯Ag in H₃P⋯AgI, but the ab initio value for the other measure of binding strength for H₃N⋯CuI (D_e = 168 kJ mol⁻¹) is significantly larger than that (116 kJ mol⁻¹) of H₃P⋯AgI. The increase, δi_c = 0.14, in the ionicity of the Cu–I bond when H₃N⋯CuI is formed is identical to that observed for H₃P⋯AgI. We conclude that H₃P⋯AgI and H₃N⋯CuI are very similar in their properties: both are strongly bound, both have similar changes in the ionicity of the M–I bond when the free MI molecule is subsumed into the complex, but the bond length r₀(M–I) is effectively unchanged in both by this process.

Several complexes involving hydrogen bonds and halogen bonds to ammonia and phosphine have been described elsewhere, namely H₃P⋯HI,²⁵ H₃N⋯HI,²⁶ H₃P⋯ICl²⁷ and H₃N⋯ICl.²⁸ All have C_3v symmetry, with all atoms but the three H atoms of PH₃ or NH₃ lying on the C^a₃ axis and therefore all are isomorphic with H₃P⋯AgI. The hydrogen-bonded analogues H₃P⋯HI and H₃N⋯HI have also been discussed in a detailed review,⁵⁶ where it is concluded, based on several indirect observations, that there is little evidence of significant charge rearrangement or HI bond lengthening in these two complexes. Both are weakly bound, having quadratic force constants F₂₂ = F_P⋯H or F_N⋯H of 3.4 N m⁻¹ and 7.2 N m⁻¹, respectively. These values are more than an order of magnitude smaller than those of H₃P⋯AgI and H₃N⋯CuI when F_P⋯Ag or F_N⋯Cu are calculated from the centrifugal distortion constant D⁰_J by means of eqn (3), the more accurate method for strongly bound complexes. The related halogen-bonded H₃P⋯ICl²⁷ and H₃N⋯ICl²⁸ have F₂₂ = F_P⋯I = 20.8 N m⁻¹ and F₂₂ = F_P⋯I 30.4 N m⁻¹, respectively, when obtained by means of eqn (4). As indicated earlier, the larger is F₂₂ relative to F₁₁, the more serious will be its underestimation when eqn (4) is used. This underestimation is likely to be negligible for H₃P⋯HI and H₃N⋯HI, but it is possible that the values of F₂₂ for H₃P⋯ICl and H₃N⋯ICl will both be somewhat larger (but only by a few %) than those reported previously. Clearly, the halogen-bonded complexes H₃P⋯ICl and H₃N⋯ICl are significantly more strongly bound than the hydrogen-bonded species H₃P⋯HI and H₃N⋯HI (when using the F₂₂ criterion) but less so than H₃P⋯AgI and H₃N⋯CuI. According to a method of estimating electric charge redistribution from the changes in the I and Cl nuclear quadrupole coupling constants,^27,28,57 there is a net movement of 0.15e (where e = electronic charge) from Cl to I in ICl when each of H₃P⋯ICl and H₃N⋯ICl is formed, thereby suggesting similar charge movement to that observed in each of H₃P⋯AgI and H₃N⋯CuI.

Acknowledgements

The authors thank the Engineering and Physical Sciences Research Council (UK) for a postgraduate studentship awarded to S. L. S. and project funding (EP/G026424/1). N. R. W. and D. P. T. thank the Royal Society for University Research Fellowships. A. C. L. is pleased to acknowledge the University of Bristol for the award of a Senior Research Fellowship.

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Footnotes

† Electronic supplementary information (ESI) available: All underlying data are provided as electronic supplementary information accompanying this paper. See DOI: 10.1039/c6cp03512d

‡ Present address: Chemistry Department, 360 Parker Building, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada.

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