J . Phys. Chem. 1988, 92, 2145-2149 show this mode to be a long-wavelength skeletal vibration, and its frequency (263 cm-I) seems safely removed from any indication of instability. To help place the BF vibrational frequencies in perspective, we compare the totally symmetric (Alg) frequencies with those of ethylene, trans-butadiene, and benzene in Table 111. The 2A!, frequency of BF, involving primarily double-bond stretching, is seen to be intermediate between the essentially pure double-bond stretching frequencies for ethylene and butadiene and the CC stretching frequency for benzene. The other BF AI, mode, with a frequency (610 cm-I) well below that for the "single-bond stretch" in tram-butadiene, is a pure radial breathing mode, having some double- as well as single-bond stretching character. The lA, mode at 972 cm-I is of particular interest. Note that the atomic motion is wholly tangential and perpendicular to the great circle planes containing the C=C bonds. Note also that the value of 0.31 for (vItvz) is just cos 2n/5. These results imply a rigid rotary oscillation of each pentagonal ring about its local fivefold axis, an interpretation that is confirmed by the O.O"/A entry for the C-C-C angle distortion. The -1.00 entry for (vltv6) indicates that all pentagonal rings rotate in the same direction, as illustrated in Figure 1. This maximizes the distortion of the
2145
angles (all C-C=C) in the hexagonal rings. Identical rotation of linked pentagonal rings is a necessary feature of the A, symmetry species but is not characteristic of other modes dominated by pentagonal ring rotation. In order of increasing frequency and angle distortion these are the 1F2g,3H,, and increasing C-C=C 2F,, modes at 591, 706, and 865 cm-I, respectively. The ring oscillation described above is not unique to BF. Independent ring oscillation coordinates can be defined for graphite and for many hypothetical spheroidal carbon clusters. We do not know how important these coordinates are, however, in describing the actual vibrational modes of these molecules. Acknowledgment. We are grateful to Prof. Harry F. King and James McIver of the Chemistry Department at SUNY-Buffalo for several helpful comments concerning this paper. The research at Brookhaven National Laboratory was carried out under Contract DE-AC02-76CH00016 with the U S . Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. Part of the work was performed at Canisius College, with the aid of a Cottrell College Science Research Grant provided by the Research Corporation. Registry No. Buckminsterfullerene, 99685-96-8.
Infrared Spectra of HNF,, NF,, PF,, and PCI3 and Complexes with HF in Solid Argon Robert Lascola, Robert Withnall, and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: August 10, 1987)
Infrared spectra of HNF2, NF,, PF3, and PCl, and their complexes with HF were observed in solid argon matrices. An asymmetric structure was deduced for (HNF2), based on comparison with HNF, spectra. Complexation of these bases with H F produced the following bands: HNF,, 3681 cm-' ( u s ) and 516 and 498 cm-I (vc); NF3, 3913 cm-' (v,) and 246 cm-' ( v p ) ; PF3, 3890 cm-' (v,) and 258 cm-' (vt); PCl,, 3868 cm-I (vs). The spectra suggested the formation of a hydrogen bond between HF and the N or P lone pair for all four complexes. A direct correlation between proton affinity and vs was found for fluoro-substituted amines, allowing prediction of the HNF2 proton affinity as 163 A 5 kcal/mol and that of NHzF as 181 f 5 kcal/mol. Basicity trends in nitrogen and phosphorus compounds are discussed.
Introduction
The study of HNF, (difluoramine) and NF3 and their hydrogen-bonded complexes with HF is interesting both in its own right and in view of recent studies in this laboratory. There is very little spectral information on HNF,, an explosive c o m p o ~ n d . ' Also, ~ investigation of the complexes continues an ongoing study of complexes between HF and substituted amines, including NH3,5 NHZF$ NH20H,' NHx(CH3)3-x,8and NHzNH2.9 This study completes the series of increasing fluorination starting with NH, and ending with NF, and creates the possibility of studying the effects of fluorine substitution on a central atom. The HNF, dimer spectrum is also interesting, as HNF2 is related to NH3, and the ammonia dimer has received considerable attention.'OJl Finally, the phosphine-hydrogen fluoride complex has been characterized and found to be weaker than the ammonia-hydrogen fluoride complex based on the base interaction with the HF acid.', (1) Comeford, J. J.; Mann, D. E.; Schoen, L. J.; Lide, Jr., D. R. J . Chem. Phys. 1963, 38, 461. (2) Christe, K. 0.;Wilson, R. D. Inorg. Chem. 1987, 26, 920. (3) Christe, K. 0. J Fluor. Chem., in press and personal communication. (4) Craig, A. D. Inorg. Chem. 1964, 3, 1628. (5) Johnson, G. L.; Andrews, L. J. Am. Chem. SOC.1982, 104, 3043. (6) Andrews, L.; Lascola, R. J . Am. Chem. SOC.1987, 109, 6243. (7) Lascola, R.; Andrews, L. J. Am. Chem. SOC.1987, 109, 4765. (8) Andrews, L.; Davis, S . R.; Johnson, G. L. J . Phys. Chem. 1986, 90, 4273. (9) Lascola, R.; Withnall, R.; Andrews, L. Inorg. Chem., submitted for publication. (10) Nelson, Jr., D. D.; Fraser, G. T.; Klemperer, W. J Chem. Phys. 1985, 83, 6201. (11) Suzer, S.; Andrews, L. J . Chem. Phys. 1987, 87, 5131.
0022-3654/88/2092-2145$01.50/0
Proton affinitiesI3 and size effects suggest that this trend will reverse for NF3 and PF3, and it is of chemical interest to make this comparison. Experimental Section
The vacuum and cryogenic techniques for matrix isolation experiments have been described previo~sly.'~,'~ HNF, synthesis was based on the methods of Parker and FreemanI6 and Christe and Wilson." The first step was to convert urea to difluorourea. Three grams of urea (Aldrich, reagent grade) was dissolved in 35 mL of H 2 0 in a 100-mL three-neck round-bottom flask. A Teflon sparge tube was introduced through one port, a thermometer was introduced through another, and the third held a Tygon exit tube. The flask was immersed in an ice-water bath, and the exit tube led to an acidic, aqueous KI solution. An Ar/F2 gas mixture (approximate ratio 211) was passed through the sparge tube at approximately 3 mmol/min until 100 mmol of F, was used; during this step the KI solution turned dark red. The difluorourea solution was then acidified and heated to liberate H N F 2 and C 0 2 in the same three-neck flask. One port admitted Ar carrier gas, one held a Pyrex dropping funnel containing 6 mL of concentrated H,SO,, and the third led to a series (12) Arlinghaus, R. T.; Andrews, L. J . Chem. Phys.1984, 81, 4341. (13) Dorion, C. E.; McMahon, T. B. Inorg. Chem. 1980, 19, 3037. (14) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76,2875. ( 15) Andrews, L.; Johnson, G.L.; Kelsall, B. J. J . Chem. Phys. 1982, 76, 5767. (16) Parker, C. 0.; Freeman, J. P. Inorg. Synth. 1970, 12(55), 307. (17) Christe, K. 0.;Wilson, R. D., private communication.
0 1988 American Chemical Society
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1400
iioo
goo
8
700
f
so0
WflVENUMBERS WAVENUMBERS Figure 1. Infrared spectra of HNFz in solid argon: (a) Ar/HNF2 = 200/1, deposited at 12 K (b) Ar/HNF2 = 100/1 and Ar/HF = 200/1, deposited at 12 K; and (c) Ar/HNF, = 100/1 and Ar/(DF + HF) = 200/1, codeposited at 12 K.
of two cold traps maintained at -78 OC in a dry iceacetone bath and at -142 OC in a methylcyclopentane slush. The acid was added dropwise over a 0.5-h period, and evolution of gas was noted. As the flask was heated (to approximately 50 "C), the evolution increased substantially. After the evolution of gas ceased, the -142 OC trap containing liquid HNF2 and C 0 2 was connected to a Pyrex vacuum manifold and evacuated (Caution: see ref 18). The methylcyclopentane slush was replaced by a -131 "C pentane slush. The trap was momentarily evacuated many times, preferentially removing C 0 2 until the vapor pressure of the HNF2/C02 solution had decreased an order of magnitude. The H N F 2 / C 0 2sample was then held at -89 OC (butanol slush) and the vapor equilibrated in a 1.O-L glass bulb with a high-vacuum stopcock and diluted with Ar to a total pressure of 1.0 atm, thus creating a 100/1 Ar/HNF, sample. Hydrogen fluoride, NF,, PF, (Matheson), and PC1, (Aldrich) were diluted in Ar to ratios ranging from 100/1 to 200/1. Deuterium fluoride was synthesized by mixing F2 (Matheson) and D, (Air Products) sufficient to make 1 mmol of gas in a wellpassivated stainless steel can and was then diluted with argon. Samples were deposited at 3-5 mmol/h each for 4-6 h onto a 12 K CsI window; the Ar/HNF2 sample was deposited through a Teflon needle valve directly onto the cold window. Spectra were recorded with either a Nicolet 7199 FTIR (range of 4000-400 cm-', resolution of f l cm-') or a Perkin-Elmer 983 spectrometer (range of 4000-180 cm-I, resolution of f 2 cm-I) both before and after annealing of the matrix by heat cycling from 12 to 26 to 12 K. Results Experiments with hydrogen fluoride and each base will be described. HNF,. Seven experiments were performed with difluoroamine samples. Those that involved only HNF, in an argon matrix showed several strong absorptions due to C 0 2 impurity and weak (18) See warnings in ref 2, 3, and 16: HNF2 is potentially
explosive.
TABLE I: Absorptions (cm-I) and Assignments for Difluoroamine and HNF,- -HF Comdexes in a Solid Areon Matrix
assignt" gas' NHF2 (NHF2)2 HNF2- -HF HNF2- -DF 3207 u,(u(NH)) 3193 3192 3172, 3161 3207 u~(G(HNF)) 1307 1304 1321, 1315 981, 945 995 995 968 u,(u,(NF~)) 972 503 ud((~(NF2)) 500 u,(u,(NF~)) 1424 1419 1444, 1433 u,(B,(HNF)) 888 879 891, 868 905 905 3681 2705 4HF) 5 16, 498 vc(HF) Fundamental assignments and gas-phase values from ref 1.
H20absorptions. Other bands seen include an intense band at 879 cm-' (labeled vg), strong, sharp bands at 1419 ( v 5 ) , 1304 ( v 2 ) , and 968 ( v 3 ) cm-', and weak bands at 3182 ( v l ) and 503 ( v 4 ) cm-]. Also seen were bands at 3172 and 3161 cm-' ( v l (dimer)), 1444 and 1433 cm-' ( v 5 (dimer)), 1321 and 1315 cm-' ( v , (dimer)), 981 and 945 cm-' (v3 (dimer)), and 891 and 868 cm-' (v6 (dimer)). These and other bands are listed in Table I. Selected parts of the HNF, spectrum are shown in Figure la. Addition of H F to more of the same sample produced characteristic absorptions due to H F monomer and polymer, as well as i m p ~ r i t i e s . ' ~Bands that appear only on codeposition of H F and HNF, include a strong, sharp peak at 3681 cm-' ( v , ) ( A = 0.4, full width half-maximum (fwhm) = 5 cm-I), a sharp doublet at 516 and 498 cm-' ( v c ) , weak bands at 3207 and 995 cm-' ( v I c and v3'), and a strong band at 905 cm-' ( v 6 ' ) . These bands are shown in Figure 1b. Substitution of D F for H F resulted in the appearance of the DF counterparts of the characteristic H F bands mentioned above, which also appeared due to H F impurity. New absorptions include a strong band at 2705 ( v , ) ( A = 0.45, fwhm = 5 cm-') and 905 cm-' (v6') and weak bands at 3207 and 995 , can be seen in Figure IC. cm-' ( v I c and q C )which NF3. Deposition of an Ar/NF, sample produced the following absorptions: very strong bands at 1027 and 898 cm-' ( v l and v , ) , (19) Andrews, L.; Johnson, G. L. J . Phys. Chem. 1984, 88, 425.
HNF,, NF3, PF3, and PC13 and H F Complexes
1 IdHF m
;1
3400 3800 3$00 3k00 WAVENUMBERS Figure 2. Infrared spectra of Ar/AX3 = 200/1 and Ar/HF = 200/1 samples codeposited at 12 K: AX3 = (a) NF3, (b) PF3, and (c) PC13.
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2147
HNF, Dimer. Identification of bands due to HNF, dimer were made by examining HNF2/Ar spectra at different difluoramine concentrations. The dimer bands noted above all displayed similar growth on annealing, precluding their identification as higher polymer, and grew with increasing H N F 2 concentration relative to their monomeric counterparts. These bands also behaved similarly regardless of the presence of HF, confirming their identity as HNF2 bands. The bands were assigned to specific vibrational modes by their proximity to HNF2 fundamental bands, which were in turn assigned by a straightforward comparison to gas-phase spectra. I Owing to the strong electron-withdrawing ability of its fluorine substituents, H N F 2 is reasonably acidic. The simplest possible structure for (HNF2)2is therefore a single hydrogen bond between one hydrogen and one nitrogen atom. Another possible structure is a symmetric dimer with both protons hydrogen bonded to the nitrogen lone pairs on the adjacent submolecules (1). This structure is reasonable because of the superior proton affinity of the nitrogen lone pair compared to fluorine (see below).
1
One of the spectroscopic consequences of this cyclic structure, however, is that only the antisymmetric or out-of-phase N-H stretching motion should be observed; because of the DB symmetry of the configuration, the symmetric N-H stretch would be inactive. Instead, the spectrum shows two dimer bands near the v1 fundamental: 3172 and 3161 cm-l. This indicates that the two hydrogen bonds are nonequivalent, and the symmetric structure is not consistent with the infrared spectrum. A reasonable dimer structure with two different hydrogen bonds is the one that involves a hydrogen-nitrogen and a hydrogenfluorine interaction (2). This structure explains the two N-H
4600
sharp bands at 648 and 497 cm-I ( v 2 and u4), and bands at 1918 and 1791 cm-I (Y, + v3 and 2vJ. Codeposition experiments with A r / H F samples produced, besides the HF system absorptions, strong new bands at 3913 (vS), 381 1 (v=), and 909 cm-I ( Y ~ and ~ ) a weak band at 246 cm-' (Ye). PF, and PC13. Spectra of PF3 in an argon matrix displayed the following PF3 bands: very strong absorptions at 883 and 849 cm-' (vl and v3), a sharp band at 486 cm-' (vz), and a mediumstrength band at 348 cm-' (v4). Resolvable bands which appeared during codeposition experiments with HF include a strong, sharp band a t 3890 cm-' (v,) and a medium, sharp band at 258 cm-I ( Y e ) ; D F substitution produced only one new band at 2854 cm-' (VJ.
In PC13experiments, only a strong unresolved doublet near 500 cm-' was observed, as the other bands were below the range of the spectrometer. Experiments with PC13 and HF produced new absorptions a t 3868 and 3858 cm-' (vJ, and PC13 and D F gave counterparts at 2838 and 2828 cm-I. Spectra of NF,/HF, PF3/HF, and PC13/HF are compared in Figure 2.
Discussion Identification of the absorptions mentioned above as belonging to a 1:l or 1:2 base- -HF complex or base dimer will be discussed, and bonding and structure of these complexes will be considered. Those bands classified as 1 :1 or 1 :2 product bands appear only on codeposition of the base (HNF,, NF3, PF3, or PCl,) and HF; they are not present when only one of the two reagents are present. Therefore, the bands must be due to a base- - H F complex; this identification is supported by the predictable frequency shifts that occur on D F substitution.
2
stretches seen and also predicts some other features' seen in the dimer spectrum. For instance, one would expect the N-F stretches of submolecule a to be blue-shifted, as they were for the HNF2-- H F complex (see below), but also for the N-F stretches of b to be red-shifted because of the interaction with the fluorine atom. This is in fact what occurs; one finds dimer bands at 981 and 945 cm-', surrounding the v3 monomer fundamental at 968 cm-', and also at 891 and 868 cm-', around the v6 monomer fundamental at 879 cm-I. Also, because the hydrogens are involved in nonequivalent interactions, there should be two different perturbations of the N-H bending motions. Again, this is the case, as dimer bands appear at 1321 and 13 15 cm-I, blue-shifted from the v2 monomer fundamental at 1304 cm-', and at 1444 and 1433 cm-', at higher frequency than the v5 monomer fundamental at 1419 cm-I. These effects are not predicted by the symmetric structure 1. No dimer bands could be seen near the v4 fundamental at 503 cm-'. Although the nonsymmetric structure offered for the H N F z dimer does not make use of both highly basic nitrogen lone pairs, there are other structural considerations that compensate for the substitution of a less basic fluorine atom. One advantage is that the formation of a five-membered ring probably allows more favorable orientation of the protons and the lone pairs, with less strain on the structures of the submolecules, as compared to the symmetric structure. Also, the fluorine atoms are farther apart and the nitrogen lone pairs are no longer facing each other, which minimizes the electron repulsions within the dimer. This model for the HNF, dimer contrasts a proposed structure for solid HNF2,,q3 which suggests that hydrogen bonding only occurs with the nitrogen lone pair. However, the structure of the solid also reflects the tendency to assume as closely packed a
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partially remove electron density from that orbital, strengthening formation as possible, which may preclude the participation of the bond and allowing the stretching motion to occur at a higher fluorine atoms in hydrogen bonding. frequency. The blue-shifts of these bands (26, 27, and 25 cm-!) Finally, the structure of the HNF, dimer is interesting in view show the small effect that the hydrogen bond has on these HYF, of the NH, dimer. The asymmetric nature of the difluoramine modes. dimer is in accord with the experimental model for the ammonia NF,, PF3, and PCl, HF. Experiments involving NF, and dimer, an asymmetric cyclic structure that involves one hydrogen H F produced new bands at 3913, 3811, 909, and 268 cm-I; the from one NH, and two hydrogens from the other N H 3 submo3913-, 909-, and 268-cm-' bands all increased 10% on annealing, lecule. while the 3811-cm-' band increased 40%. The first three bands H N F , HF. The new product bands seen in the HNF,/HF are thus assigned to the 1:l complex, and the last is assigned to spectra were identified as the 1:l complex product on the basis the 1.2 complex. PF, and PCI, experiments produced too few of relative warm-up behavior. All the bands seen-368 1, 3207, bands for meaningful relative growth comparisons; however, the 995, 905, 516, and 498 cm-l-grew 10-20% ?n annealing and behaviors of the 3890- and 258- (PF,--HF) and 3868-cm-! changed the same amount with changing H F concentration. (PC1,- -HF) bands are characteristic of 1:l complex bands, leading Assignment of these bands was aided by substitution of DF for to a 1:l complex identification. HF, which caused large, predictable frequency shifts in some NF,, PF,, and PCl, fundamental bands were, like HNF,, easily modes and little or no shift in others. Shifting of the 368 1-cm-' identifiable from comparison to gas-phase work.2' Substitution band to 2705 cm-l in the HNF2--DF experiment led to assignment of DF for H F produced the following frequency ratios for bands of the band as vs, the perturbed H-F stretching motion in the from PF, and PCl, experiments: 3890/2854 = 1.363 (PF,); complex. The H F / D F ratio, 1.361, is typical for H F complexes 3868/2838 = 1.363 (PCI,). This leads to assignment of these and supports the assignment. The bands at 516 and 498 cm-' bands to v, for the PF3- - H F and PC1,- - H F complexes. While also shifted on DF substitution, but to a frequency below the range no NF, DF experiments were performed, the 3913-cm-' band of the instrument. However, these bands can be assigned to up, can be assigned to v, for the NF3-- H F complex based on comthe H-F librational motion, since the bands did shift substantially parison to the similar PF3- - H F and PC1,- -HF spectra. with DF substitution, and also because their frequency is appropriate for the basicity of HNF, (based on the frequency of v , ) . ~ ~ ~ As for the other product absorptions seen in these experiments, the 246- (NF,) and 258-cm-' (PF,) bands are assigned to H F All the other bands belonging to the HNF,- -HF complex (3207, librational modes for the appropriate complexes with HF. This 995, and 905 cm-I) did not show any frequency shift on DF can be done because the frequencies are quite low, as is expected substitution. This behavior characterizes these bands as belonging for complexes with very weak bases; for example, the N,- - H F to the HNF, submolecule in the complex. Their proximity to complex, which has its v, mode at 3881 cm-I, in the region of fundamental bands leads directly to their assignments as recurrent interest, also exhibits a strong mode at 262 cm-'.,, The spectively vIc, the perturbed N-H stretch, and v,' and Vg', the 909-cm-' band seen in the NF,/HF experiment is assigned to Y?, perturbed symmetric and antisymmetric N-F stretches of the the perturbed antisymmetric N-F stretch, due to its proximity HNF, submolecule in the complex. to the v3 fundamental absorption. Also, the 381 1-cm-' absorption The question of the structure of the HNF2- - H F complex inin the same experiment is assigned to vsar the stretch of the inner volves whether the fluorine substituents are sufficiently electron H F submolecule in the 1:2 complex. Perturbation effects of the withdrawing to diminish the basicity of the nitrogen lone pair. outer H F submolecule cause the inner H F to have a lower There are no experimental or theoretical studies available constretching frequency than its counterpart in the 1:l complex. cerning the proton affinity of the molecule, but it can be concluded The question of whether the hydrogen bond in the NF3--HF from the spectra that the complex contains a hydrogen bond complex involves a nitrogen or fluorine lone pair is even more between H F and the nitrogen lone pair (3). This conclusion can relevant for that complex than for the HNF,- -HF complex, beF cause of the presence of an additional electron-withdrawing I fluorine atom. Fortunately, unlike HNF,, the proton affinity of 7 NF, has been m e a s ~ r e d , 'and ~ , ~it~is clear from these studies that i the nitrogen lone pair is the most basic site of the NF, molecule. Therefore, a CJUstructure, like that for the NH3- - H F c ~ m p l e x , ~ is suggested for the NF3-- H F complex (4) and is substantiated F by observation of a single mode. 3
+
+
+
be made from the observed perturbation of the N-H stretching mode, which would not likely occur if the hydrogen bond involved a fluorine lone pair, and from the position of v, in the fluoramine series. The blue-shifts that occur in the base submolecule stretching modes (q', v3c, and v6c-see Table I) are not uncommon; for example, the HCN- -HF complex shows a 27-cm-' blue-shift in the m N stretching frequency.M This behavior can be explained in terms of either lone-pair polarization or molecular orbital theory. Because of the large electron density of the fluorine atoms, there are substantial repulsions between fluorine and nitrogen lone pairs. These repulsions are enhanced by the electron-withdrawing effect of the fluorine substituent, which causes the nitrogen lone pair to be closer to the nucleus. Formation of a hydrogen bond at the lone pair would bring electron density farther away from the atom and decrease the electronic repulsions with fluorine. This would slightly strengthen the bonds and increase the appropriate stretching frequencies. Alternately, one could theorize that interaction between the nitrogen and fluorine lone pairs in a molecular orbital scheme would give a filled antibonding MO. Then, a hydrogen bond would (20) Andrews, L.; Hunt, R. D.J . Phys. Chem. 1988, 92, 8 1 .
F
I
H !
A 4
The frequencies of the v, and ut bands attest to the highly diminished basicity of the nitrogen lone pair. The v S frequency of 3913 cm-' represents only a 6-cm-I shift from the unperturbed H F frequency in the Ar matrix, making it one of the weakest bases studied in this laboratory. In fact, its proton affinity of 148 kcal/molI3 is less than twice that of Ar (90 kcal/mol). The ut frequency, 246 cm-', is also typical of a very weakly bound complex. The one base submolecule mode seen, v3' at 909 cm-I, is blue-shifted from the v3 fundamental band at 898 cm-I. This (21) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; Wiley-Interscience: New York, 1970. ( 2 2 ) Andrews, L.; Kelsall, B. J.; Arlinghaus, R. T. J . Chem. Phys. 1983, 79, 2488. (23) Holtz, D.; Beauchamp, J. L.; Henderson, W. G.; Taft, R. W. Inorg. Chem. 1971, 10, 201.
HNFz, NF,, PF,, and PCl, and H F Complexes
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2149
TABLE 11: Proton Affinities and v, Perturbations for Fluoro-Substituted Amines PA, kcalimol Av,,cm-' a NH3 NH2F NHF2 NF3
203b [ 18
1 ] c 3 d
[163Id 148'
U
H
F
D
T
877 530 238 6
"Difference between observed v, and HF fundamental at 3919 cm-' in solid argon, ref 5 (NH,) and 6 (NH2F). bCollyer, S.M.; McMahon, T. B. J . Phys. Chem. 1983, 87, 909. CTheoreticalcalculation, 191 kcal/mol. Johansson, A.; Kollman, P. A.; Liebman, J. F.; Rothenberg, S. J . Am. Chem. SOC.1974, 96, 3750. d f S kcal/mol. 'Reference 13.
is the same effect as for the N-F stretching modes in the HNF2--HF complex, and the explanations offered in the discussion of the latter complex are also applicable for this complex. On the basis of spectral similarities between the three complexes, the PF3--HF and PC1,- -HF complexes are assigned the same C,, structure as the NF3- -HF complex, which is again supported by observation of a single ut mode. The similar v, (3890 and 3868 cm-', respectively) and ut frequencies (258 cm-' for PF3) attest to the weak nature of these bases, due to the electron-withdrawing fluorine and chlorine substituents. Bonding Trends. Strength of Dimerization. Comparison of the perturbed base submolecule frequencies in the HNF2- -HF complex to those seen for the H N F 2 dimer show that they are consistent in indicating that the HF complex is stronger than the dimer; the greatest differences of the dimer bands from their associated fundamentals are 21 ( Y , ) , 23 ( v , ) , and 1 1 (v6) cm-', as opposed to 26,27, and 25 cm-' for the H F complex. This agrees with the fact that HF is more acidic than HNF2. This consistency has been seen before with NHzOH, except that the hydroxylamine dimer was more strongly bound than the N H 2 0 H --HF complex.' Proton Affinities. Study of the HNFz and NF, complexes with HF in this laboratory completes the series of increasing fluorine substitution starting with NH, and ending with NF3.596 This allows examination of the effect of this substitution on the basicity of the nitrogen lone pair, since the hydrogen bond with HF is formed at that site in all four complexes. Also, prediction of the proton affinities of HNF, and N H 2 F from the data for the other molecules is possible. The results from these experiments are summarized in Table 11. It can be seen that the perturbation of the H F stretching frequency decreases drastically as each fluorine is added. It is also evident that each fluorine atom added does not have as much of an effect on the basicity of the molecule as the previous fluorine atom (the differences in Y , perturbation are 347, 292, and 232 cm-I). Both observations are in accord with chemical intuition. Infrared spectra for the four complexes NH3, NH2F, NHF,, and NF, with HF are compared in Figure 3. A simple linear regression analysis was done with data from NH, and NF, to predict proton affinities for H N F z and NH2F. Assuming a linear relationship between proton affinity and us, the Av, value of 238 cm-' predicted a HNF, proton affinity of 163 f 5 kcal/mol and a N H 2 F proton affinity of 181 f 5 kcal/mol. The assumption of linearity is justified for this series of complexes because they all involve a similar basic site, the nitrogen lone pair. If lone pairs of other atoms or delocalized r-systems were also involved, the assumption would be harder to justify. Also, a similar study involving the NH,(CH,),-, series found a linear relationship between proton affinity and us perturbation,* supporting the use of such a relationship in this case. Nus P Basicity Trends. Comparison of the NH3,5PH3,lZNF3, PF,, and PCl, spectra shows interesting trends in nitrogen and phosphorus basicity. For example, ammonia is more basic than phosphine (v,(NH,) = 3041 cm-l, v,(PH,) = 3627 cm-I), but PF, is more basic than NF, (v,(PF,) = 3890 vs 3913 cm-' for v,(NF,)) based on shifts in v,(HF) modes. The inductive effect of fluorine reduces the basicity of both NF, and PF, relative to N H , and PH,, respectively, but this effect is much less for PF,, which has the more diffuse lone pair. Since the inductive effect of chlorine
a * i
3600 3400 3200 3b00 WRVENUMBERS Figure 3. Infrared spectra of fluoramine + hydrogen fluoride complexes in solid argon; (a) Ar/NH, = 200/1 and Ar/HF = 500/1 codeposited at 12 K (b) Ar/NHp = 400/1 and Ar/F, = 200/1 codeposited at 12 K and photolyzed for 30 min; (c) Ar/HNF2 = 100/1 and Ar/HF = 200/1 codeposited at 12 K and (d) Ar/NF, = 200/1 and Ar/HF = 100/1, codeposited at 12 K. Lib00
3800
is less than fluorine, the basicity of PH, is reduced less in PCl, than in PF,, and PC1, is expected to be more basic than PF,. This is substantiated by the v,(HF) modes in the complexes (PCl,, v, = 3868 cm-I). A similar effect has been found for HF complexes with N H 2 F and NH,Cl; chloramide is more basic than fluoroamide based on their amide lone pair interactions with the acid H F (NH2F, V, = 3389 cm-l and NH2C1, Y , = 3311 cm-').6
Conclusions Spectra of HNFz, NF,, PF,, and PCl, and their complexes with H F were observed in a solid argon matrix. For each of these complexes, a hydrogen bond was formed between the H F and either the nitrogen or phosphorus atom. A linear relationship between proton affinity and perturbation of the v, stretching mode was used for the series NH,, NH2F, NHF,, and N F 3 to predict NHF, and NH2F proton affinities. Also, it was found that PF3 is more basic than NF,, even though NH, is more basic than PH,, based on spectra of their HF complexes, which is consistent with gas-phase proton affinities. PCI, was found to be more basic than PF, owing to the weaker inductive effect of chlorine. Although HNF, is acidic, base submolecule modes show that (HNF2)2is less strongly interacting than HNF2- -HF. Acknowledgment. We gratefully acknowledge support from N S F Grant C H E 85-1661 1 and helpful correspondence with K. 0. Christe on H N F z synthesis. Registry No. HNF2, 10405-27-3; NF,, 7783-54-2; PF,, 7783-55-3; PCI,, 7719-12-2; HF, 7664-39-3; Ar, 7440-37-1.
