5594
J . Phys. Chem. 1987, 91, 5594-5598
Infrared Spectra of Cyanogen Halide Complexes with Hydrogen Fluoride in Solid Argon Rodney D. Hunt and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: April 7, 1987)
Cyanogen chloride was condensed at 12 K with HF in excess argon, producing two 1:l complexes of the form CIC=N--HF (1) and HF- -ClC=N ( 2 ) . Increasing the H F concentration produced a 1:2 complex, CICEN- -(HF),, while increasing the CICEN concentration produced a 2: 1 complex, CIC=N- -HF- -CIC=N. The H F submolecule stretching frequency for the major primary 1:l complex (1) was observed at 3597 cm-I, and a single HF librational mode appeared at 602 cm-I. These fundamentals are comparable to the H F modes for alkyl cyanide complexes. In sharp contrast, the second 1:l complex (2), which is similar to the HF- -CIF and HF- -C12 complexes, displayed a HF stretching frequency at 3912 cm-I. Similar results were obtained for cyanogen bromide and cyanogen iodide complexes with hydrogen fluoride and deuterium fluoride. In addition, the photolysis of hydrogen cyanide and fluorine produced the complex F e N - -HF with no evidence for a complex analogous to 2. The H F stretching and librational modes for this complex were observed at 3662 and 553 cm-', respectively. Similarly, the reaction between cyanogen and HF formed only the N = C e N - -HF complex. The HF modes for this complex were observed at 3757 and 460 cm-I, which indicate a weaker interaction than found for the cyanogen halides and HF.
Introduction Hydrogen bonding and Lewis acid-base interactions have been studied extensively due to their importance in determining the physical properties of a wide range of complexes. Simple complexes with hydrogen fluoride serving as a Lewis base or Br~nsted acid have been widely used as models for more complicated systems. Thus HF can form two distinctly different types of complexes, which have been characterized in numerous gas-phase investigations. Microwave studies have obtained evidence for the formation of linear, hydrogen-bonded HCEN- - H F and NE C e N - -HF complexes.'p2 In sharp contrast, investigations using molecular beam electric resonance spectroscopy have determined the structures3a4of C12 and C1F complexes with HF to be antihydrogen-bonded HF- -el2 and HF- -ClF. On the basis of these gas-phase results, HF possesses the unique ability of serving as a Lewis base as well as a Bransted acid in complexes with cyanogen halides. The matrix isolation technique is especially suited for the characterization of these types of complexes. The structure and bonding of HF complexes with such molecules as RCN,sv6H2NCN,' el2, and C1F8 have been examined by using the matrix technique and infrared spectroscopy. The purpose of this study is to examine the different complexes between HF and cyanogen halides and to investigate the competing bonding effects between the cyanogen and halogen subunits in the complexes. Infrared spectra of complexes of hydrogen fluoride with cyanogen halides will be described below.
bromide (Aldrich), and cyanogen iodide (Kodak) were each purified by evacuation at 77 K to remove volatile impurities. Deuterium fluoride was prepared by reacting F2 (Matheson) with D2 (Air Products) at low pressures in a passivated stainless steel vacuum system. Hydrogen cyanide was synthesized by the method previously described.s Cyanogen was prepared from silver cyanide obtained from a solution of K C N by precipitation with AgN03. After drying, the AgCN was heated in vacuo at 34C-380 "C, and the evolved cyanogen was collected at 77 K. The acid and base samples with the exception of cyanogen iodide were diluted between 1OO:l and 400:l mole ratios with argon (Air Products). ICN vapor was sublimed from a glass finger at room temperature directly into the A r / H F stream. Reagent samples were codeposited on the CsI window at approximately 14 total mmollh for 4 h. The samples following deposition were annealed to between 20 and 23 K for 10 min and then recooled to 12 K. The matrices containing H C N and F2 were photolyzed with a mercury arc followed by sample annealing to 32 K with the techniques developed previo~sly.~Spectra were recorded before, during, and after sample preparation, photolysis, and annealing. Blank samples were prepared and annealed for each reagent separately.
Results Fourier transform matrix infrared spectra of H F and HF/DF mixtures with cyanogen halides and cyanogen will be described in turn. Cyanogen Chloride. Four experiments were performed with cyanogen chloride and hydrogen fluoride at high dilution in argon codeposited at 12 K. In the most concentrated samples, the spectrum, displayed in Figure la, showed HF monomer, dimer, and trimer bands (labeled HF, D and T),Io absorptions due to water and H20-HF complexes (labeled WC),"bands due to nitrogen complexes (labeled N),IZand strong precursor absorptions. New product absorptions included an extremely strong and sharp band at 3597 cm-' ( A = absorbance > 2.0, labeled vs) with satellite bands at 3647 (A = 0.16) and 3635 cm-' (A = 0.22), a strong sharp absorption (labeled V I ) at 602 cm-' ( A = 0.74), and a weaker band (labeled 2~1)at 1104 cm-I (not shown). Several weaker product absorptions involving HF were observed at 3912 (labeled aVs), 3694 ( V s b ) , 3520 ([VS]), 3361 (VM), 3336 (Vsa), 670 (Via), 618 ( [ v , ] ) , and 539 Cm-' (Vlb). In addition, a strong, sharp satellite was observed at 2246 cm-' above precursor vl at 2209 cm-', a weak product absorption was observed at 808 cm-I above the 2u3 mode of ClCN at 792 cm-I, and no absorption was detected above v 2
Experimental Section The vacuum, cryogenic, and spectroscopic techniques used in these experiments have been described p r e v i ~ u s l y .Infrared ~~~ spectra were recorded on a Nicolet 7199 Fourier transform infrared spectrometer at a 1-cm-' resolution in the region 4000-400 cm-'. A single beam spectrum of the CsI window at 12 K was recorded and ratioed as a background to a single beam spectrum of each experiment. All reported frequency values were rounded to the nearest wavenumber. Hydrogen fluoride (Matheson), deuterium fluoride, hydrogen cyanide, cyanogen, cyanogen chloride (Matheson), cyanogen (1) Legon, A. C.; Millen, D. J.; Rogers, S.C. Proc. R. SOC.London, A 1980, 370, 213. (2) Legon, A. C.; Soper, P. D.; Flygare, W. H. J. Chem. Phys. 1981, 74, 49%
(3) 1632. (4) 51 15. (5) (6)
Baimhi, F. A,; Dixon, T. A,; Klemperer, W. J. Chem. Phys. 1982, 77, Novick, S. E.; Janda, K. C.; Klemperer, W. J. Chem. Phys. 1976,65,
(9) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1985, 82, 4442. (10) Andrews, L.; Johnson, G. L. J. Phys. Chem. 1984,88, 425. (11) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1983, 79, 3670. (12) Andrews, L.; Kelsall, B. J.; Arlinghaus, R. T. J. Chem. Phys. 1983, 79. 2488.
Johnson, G. L.; Andrews, L. J . Am. Chem. SOC.1983, 105, 163. Johnson, G. L.; Andrews, L. J . Phys. Chem. 1983, 87, 1852. Davis, S. R.; Andrews, L. J. Mol. Spectrosc. 1985, 111, 219. (7) Hunt, R.D.; Andrews, L. J. Phys. Chem. 1987, 91, 2751. (8) Hunt, R. D.; Andrews, L., submitted for publication.
0 1987 American Chemical Sociatv 0022-3654/87/2091-5594%01.50/0 , , .-, I
-
Cyanogen Halide-Hydrogen Fluoride Complexes
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5595
'vs
DF
OF
( b1 x,
oo
sbao
3800
3400
WAVENUMBERS
$too
sboo
fii-700
I
Figure 1. FTIR spectra of cyanogen chloride-hydrogen fluoride samples: (a) after codeposition of 27 mmol of Ar/ClCN = 200:l and 27 mmol of Ar/HF = 150:l at 12 K for 4 h; (b) after warming to 23 K and recooling to 12 K.
at 718 cm-I. The matrix was annealed to 23 K and recooled to 12 K, and another spectrum was recorded (Figure lb). Hydrogen fluoride dimer and trimer as well as impurity complexes increased. The primary HF product absorptions from Figure l a increased in intensity by 20%, and the secondary product bands involving HF grew by a factor of 2. In less concentrated samples, these secondary product absorptions did not appear until the matrix had been annealed. Two additional experiments were conducted with D F / H F mixtures. The absorptions observed in the HF experiments were again present. In the spectrum from the most concentrated D F / H F sample (>90% DF), displayed in Figure 2a, new absorptions specific to the D F system appeared at 2649 cm-' ( A > 2.0; labeled v,) with satellites at 2684 and 2678 an-',at 2598 cm-' (labeled [v,]), and a t 2247 cm-' above ClCN at 2209 cm-'. A weak new band was observed at 800 cm-'above 2v3 of ClCN, and a weak shoulder was observed at 390 cm-' above v3 at 385 cm-I. Additional D F product absorptions at 863 cm-' (labeled 2vl) and 448 cm-' (labeled q), and absorptions due to 1:2 D F complexes, are listed in Table I. Again, annealing the sample to 23 K (Figure 2b) produced the largest increase in intensity in the secondary 1:2 D F product absorptions. Cyanogen Bromide and iodide. Parallel experiments were conducted with cyanogen bromide and iodide over the same concentration ranges and annealing conditions employed for cyanogen chloride. The product band positions are listed in Table I. The major difference between ClCN and the other cyanogen halides is the increase in the number of us and u, sites for the larger cyanide bases and the increase in yield of sharp av, bands at 3905 and 3891 cm-' with BrCN and ICN. The absence of H C N from the ICN experiments is noted. The unusually large separation between split v, modes in the BrCN complex is probably due, in part, to a matrix cage effect. Annealing the BrCN matrices to 22 K for 10 min produced the same growth pattern as previously described for ClCN. However, annealing the I C N matrices to 17 and 2 3 K failed to produce any significant changes in the spectrum due to the extensive amount of aggregation that occurred
lo1
1 2450
2600
2850
2500
2350
2200
WAVENUMBERS Figure 2. FTIR spectra of cyanogen chloride-deuteriumfluoride samples: (a) after codeposition of 29 mmol of Ar/ClCN = 200:l and 31 mmol of Ar/(DF HF) = 150:l at 12 K for 4 h; (b) after warming to 23 K and recooling to 12 K.
+
TABLE I: Absorptions (cm-') Produced on Deposition of Cyanogen Halides and Hydrogen Fluoride in Excess Argon
ClCN + ClCN + BrCN + BrCN + ICN + ICN + HF DF HF DF HF DF 3912 3694
2871 2712
3905 3685
2865 2712
3597
2649
3520
2598
3635 3577 3438
2675 2636 2542
3361 3336 2250 2246 1104 808 670 618 602
2488 247 1 2250 2247 863 800 494 459 448
3332 3311 2229 2215 1128
2468 2453 2229 2216 815
678 645 573
475 428
616
455
539
397
547 541
3891 3696 3678 3609 3616 3502 3491 3291 3269 2207 2193 1070
2856 2707 2696 2659 2663 2586 2575 2444 2429 2207 2194 825
692 66 1 590 580 538
517 486 438 430
assiant
usa
u I C(site)
u I c (site)
VI
uI (site) bI' Ylb
during sample deposition. In the more concentrated samples of BrCN and ICN, the Vsb, [v,], v,,, via, [ u l ] , and Vlb bands were considerably stronger. HCN F2. Several molecular fluorine codeposition and photolysis experiments were conducted with H C N and H C N / D C N mixtures. The product band positions are listed in Table
+
5596
Hunt and Andrews
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
TABLE II: Cyanogen Fluoride-Hydrogen Fluoride Complex Absorptions (em-') from Hydrogen Cyanide and Fluorine in Excess Areon following Photolvsis and Sample Warming HCN + F, DCN F2 assignt
+
3707 3662 3512 2355 1 IO7 999 553 459
"sb
"s
2694 "sa
2356 1107
vic
(2318)"
v > (io76j
281 VI
459
vjc (456)
" Values for modes of uncomplexed FCN in solid argon; ref
13 and
22.
i"
TABLE III: Absorptions (cm-') Produced on Deposition of Cyanogen and Hydrogen Fluoride in Excess Argon
(CN),
+ HF
(CN),
3757 3783 3764 3740 3692 3524 3537 3515 2165 805 635 630 508 498 460 457 443 440
+ DF
2758 2777 2763 2747 2714 2596 2604 2590 2165 645
assignt YS
,xhjj
us (site) us (site)
vs (site) "sb
$4 '
ysa
uSa (site) uSa (site) vjC (2154)"
2Ul "la
Yla
508 3686 3386 3366
v; (503)" vlb VI VI
v1 (site)
v i (site)
"Precursor values; this work and ref 18. bobserved in separate farIR experiment; see ref 9.
+
11. In the most concentrated H C N F2sample, the spectrum following deposition showed weak HF and water absorptions and very strong H C N absorptions. Following photolysis, strong new H F product absorptions at 3662 (v,) and 553 (vI) cm-I and weaker precursor satellites were observed at 2355 (vlc), 1107 ( v z c ) , and ~ ) When the matrix was annealed to 32 K and then 459 ( v ~cm-'. recooled to 12 K, these HF photolysis products decreased in 'intensity by a third, and weak HCN- -HF bands and new 3707and 3512-cm-I bands were observed. In addition, two other photolysis products, HFCN and HFCNF, were observed and will be discussed in a later paper.13 In the experiments performed with HCN/DCN mixtures, the absorptions observed in the HCN experiments were again present. After photolysis, the new absorptions specific to the D F system were observed at 2694 (v,) and 2356 cm-' (vlc). Cyanogen. A series of similar studies was performed with cyanogen, and the results are presented in Table 111. In the experiment with the most dilute samples, the spectrum, displayed in Figure 3a, revealed a strong v, band at 3757 cm-' and a doublet (labeled vI) at 460 and 457 cm-' along with several satellite bands. In addition, perturbed base submolecule modes were observed at 2165 (qC)and 508 cm-' (vqC). Most of these bands doubled on annealing to 21 K, and several new absorptions (labeled v, vsb, via, and vlb) appeared as shown in Figure 3b. With more concentrated samples, v, and v1 bands were much stronger, and the higher order product bands were observed on sample deposition. Annealing the samples had very little effect on the v, and vI absorptions while the other product bands increased markedly. Discussion The new product bands will be identified, and vibrational assignments will be made. The structures and bonding of the (13) Hunt, R. D.; Andrews,
UC
L. Inorg. Chem., in press
i
4 00
WAVENUMBERS
600
$00 $00 WAVENUMBERS
Figure 3. FTIR spectra of cyanogen-hydrogen fluoride samples: (a) after codeposition of 28 mmol of Ar/(CN), = 200:l and 28 mmol of Ar/HF = 400:l at 12 K for 4 h; (b) after warming to 21 K and recooling to 12 K.
cyanogen halide complexes will be compared to other similar complexes. Also, the competing effects between the cyanogen and halogen subunits will be examined. Identification. The new product absorptions listed in Tables I, 11, and I11 were not observed in argon matrix samples of the cyanogen compounds or hydrogen fluoride alone; however, these bands were produced with high yields when the reagents were mixed during the condensation process. Four groups of HF product absorptions can be identified on the basis of sample concentration, annealing behavior, and band position. Absorptions of the first group were strong after sample deposition regardless of the concentration, and they increased in intensity by approximately 20% on sample annealing. Absorptions in the second group were very weak after reagent condensation at all concentration levels, and they increased by less than 2-fold following sample annealing. Bands of the third and fourth groups were weak after sample deposition, and they more than doubled in intensity on sample annealing. The first group of absorptions with labels v, and vl were strong after sample deposition, indicating that the bands belong to the major product species. This group of bands maintained constant relative intensities over the wide range of sample concentrations and annealing conditions. The first group of absorptions can be assigned to the major 1:1 complex between the cyanogen base and H F which has the nitrile-bonded structure, based on agreement with the spectra of HCN and CH3CN complexes. The absorption (labeled av,) exhibited the same concentration dependence as observed for the first group, which indicates that this absorption can be attributed to a second 1:l complex most likely with the
Cyanogen Halide-Hydrogen Fluoride Complexes anti-hydrogen-bonded structure 2. The third group of HF product absorptions with labels u, VSb, ula, and Vlb displayed a higher order concentration dependence on HF, which is characteristic of the 1:2 complex, 3.s,13The fourth band group, with labels [us] and [vi], appears to have a higher order dependence on the cyanogen halide, which characterizes a 2:l complex, and 4 is the proposed structure. It should be noted that only HF complexes 1 and 3 were observed with F C N produced by photolysis of HCN/F2 mixtures. H XCEN:-
\
- HF
\ r
F'
F- - X C E N :
2
1 XCEN,:
XCZN:--HF--XC=N:
H- -F
4
3
Assignments. The strong, sharp primary product absorptions (labeled us) have similar D F counterparts with H F / D F ratios of 1.357-1.362, which is characteristic of the fundamental vibration of the H-F ligand in the primary complex 1. These bands are appropriately displaced from the 3919-cm-I HF fundamental in solid argonlo due to the hydrogen-bonding interaction. The ul bands are assigned to the two degenerate librational modes of the HF submolecule in the primary 1:l complex arising from the two rotational degrees of freedom of the diatomic. In the case of ClCN-HF, a single sharp 602-cm-' absorption was observed with a H F / D F ratio of 1.348 and an overtone band for the uI mode was seen at 1104 cm-I with an overtone/fundamental ratio of 1.834. In the case of (CN),-HF, the ul mode is split into a 460-, 457-cm-' doublet with H F / D F ratios of 1.360 f 0.001. The splittings on the vI mode must be due to matrix site asymmetry, since the potential surface for the two librations must be the same for this linear complex.2 In conjunction with these HF submolecule absorptions, product bands were identified for the perturbed u3 and u4 modes for cyanogen as well as the perturbed u1 mode for each halide complex 1 except for FCN, which revealed three perturbed F C N submolecule modes. The bands (labeled uc) displayed the same HF concentration dependence as the HF submolecule bands for the 1:1 complexes. The appearance of the perturbed u4 mode in the cyanogen-HF experiments is very interesting since the u4 mode is only Raman active for (CN),. Activation of normally infrared N2,I5C 0 2 inactive fundamentals has been observed for H2, 02,14 (Fermi resonance doublet, u I 2u2),16and C2H2 (u2 and v4)17 complexed to HF. In these complexes activation is brought about by electrical asymmetry induced in the base submolecule by the HF ligand. In the H2and 0, studies, the infrared inactive modes were slightly activated by molecular aggregation or by the matrix itself and HF served to intensify these absorptions by a factor of 10. In contrast, v4 of (CN), was not infrared activated until HF complexed with (CN),. In addition, vqCof (CN),- -HF was observed at 508 cm-', which represents a small blue shift of 5 cm-' from the gas-phase Raman value.'* An interesting interaction between the perpendicular v, and u3 modes of each submolecule was found for the ClCN- - H F and ClCN- -DF complexes as manifested in the 2u3 mode, which was intensified by Fermi resonance19 with u2. For ClCN- -HF the ul mode at 602 cm-I interacts little with u3c (presumably near 394 cm-' but not detected); however, for ClCN--DF, the uI mode shifted to 448 cm-I, interacting slightly with u3c and producing
+
(14) Hunt, R. D.; Andrews, L. J . Chem. Phys. 1987,86, 3781. (15) Andrews, L.; Davis, S . R. J . Chem. Phys. 1985,83, 4983. (16) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6353. (17) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Phys. Chem. 1982,86, 3314. (18) Warsop, P. A.; Montero, S . J . Raman Specrrosc. 1978, 7, 115. (19) Lafferty, W. J.; Lide, D. R.; Toth, R. A. J. Chem. Phys. 1965, 43, 2063. Here we follow the convention of the highest symmetric fundamental is v, (the m N stretch), the next higher is u2 (the C C 1 stretch), and the band is v3.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5591 TABLE I V Comparison of HF Stretching and Librational Modes (cm-I) for Cyanogen Halides and Cyanogen and Other Similar Complexes ~~
complex HF-CICN HF- -BrCN HF-ICN HF-Cl2 HF- -CIF (CN)2-HF C2HCl-HF C2H2-HF CH3F--HF CH3Cl-HF FCN-HF HCN-HF BrCN-HF ICN-HF ClCN-HF CH,=CHCN-HF CH3CN-HF
v, (argon) 3912 3905 3891 3902 3896 3751 3164 3145 3114 3126 3662 3626 3635, 3511 3609 3597 3566 3483
ul
(argon)
460, 457 426, 382 453,435 436, 318 553 586 616, 606, 573 590 602 624, 621 680
a weak 390-cm-l shoulder. In the overtone region 2u3cwas found at 800 cm-I for ClCN- -DF and at 808 cm-l for ClCN- -HF, both above the 792-cm-' ClCN value. This acid submolecule isotopic shift in the 2u3cbase submolecule mode is readily explained by interaction between the perpendicular uI and u3 fundamentals of each submolecule in the complex. Comparisons, Bonding, and Structure. Even though cyanogen halides are relatively simple molecules, they have three potential sites where HF can hydrogen bond to form a 1:l complex. These sites are the lone pair on the nitrile nitrogen (Le., H C N and CH3CN),4v5the lone pairs on the halides (Le., alkyl halides),I9 and the a system (i.e., C2H2and C2HC1).17 In addition, HF can serve as a Lewis base and produce a 1:l complex of the form HF- -ClCN (Le,, C12 and C1F).8 On the basis of the positions of us and vI, the multiplicity of the u1 absorptions, and the perturbations on the u1 cyanogen halide submolecule modes, the structure of the principal 1:l complex is nitrile bonded similar to the HCN-HF4 and CH3CN-HF5 complexes. The u, and uI modes, which provide the most information about bonding and structure, are compared in Table IV for the cyanogen halides and other potentially similar bases. The vS and uI modes of ClCN-HF are extremely close to those of HCN-HF. If HF were hydrogen bonded to the lone pair on the halogen or to the a system, the uI mode would be split due to the asymmetry of the complexes, based on C2H2and CH3Cl complexes with H F as example^.'^^^^ The perturbed submolecule mode of the cyanogen halides is a valuable source of information about the site of attachment of H F in the complex. One expects the largest perturbation to occur to the submolecule mode(s) that include atoms involved in the hydrogen bond. The only mode perturbed for all of the cyanogen halides was the C=N stretch, u l , which was blue shifted 37, 37, 29, and 23 cm-' from the base submolecule position for the FCN, ClCN, BrCN, and ICN complexes, respectively. For comparison, a blue shift of 28 cm-' was found for the C=N stretch in the -HF c o m p l e ~ , ~while ~ ' ~the * ~perturbation ~ ~ ~ ~ on the C=C H-Nstretch in the C2H2 complex was much less ( f 2 cm).17 This information strongly indicates that H F hydrogen bonds to the nitrile lone pair. On the basis of the perturbed ulc mode as well as us and uI, the structure of the major cyanogen halide complexes must be XCN--HF (X = F, C1, Br, and I). Note in Tables I and I1 that DF blue shifts the CEN fundamentals of all 4 XCN complexes l cm-' more than HF. This greater perturbation for DF arises from the closer average distance of D and the base owing to the smaller DF librational amplitude; this has been observed for a number of c0mp1exes.l~ In spite of the fact that a blue shift is indicated for the F-C stretching mode for the FCN- - H F complex, based on the argon (20) Arlinghaus, R. T.; Andrews, L. J . Phys. Chem. 1984, 88, 4032. (21) Andrews, L.; Hunt, R. D., J . Phys. Chem., in press. (22) Jacox, M. E., unpublished results.
5598
Hunt and Andrews
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
matrix base submolecule band p o s i t i o t ~ , 'no ~ , ~counterparts ~ were observed for the heavier cyanogen halides. The Fermi doubletlg 2v3 and v2 for ClCN under higher resolution matrix conditions reveals sharp 3:l doublets at 791.7 and 789.0 cm-' and 717.9 and 71 1.6 cm-I. The magnitudes of these chlorine isotopic splittings show that the Fermi doublet is correctly described as more ut for the lower component and more 2u3 for the upper component. However, it must be noted that the observed splitting in v2, 6.3 cm-I, is more than the 5.1-cm-I value calculated for a diatomic harmonic C-C1 oscillator. This, of course, indicates that the v2 mode has molecular symmetric stretching character and involves more than a simple C-Cl motion. Since the F substituent has a much greater effect on the m N mode in the base submolecule, it is not unreasonable to find the C-F mode perturbed more in the nitrile lone pair X C N complexes. In addition to the primary 1:1 complex, a second 1:1 complex has been identified from the positions of the v, modes. The structure of this weaker 1:l complex is approximately the same as those for the anti-hydrogen-bonded Lewis acid-base HF- -C1, and HF- -ClF complexes3v4which absorb in the same region. The av, modes for cyanogen halides and diatomic halogen complexes* are very close, as shown in Table IV. If the acid hydrogen were hydrogen-bonded to the halogen site, the red shift of the H-F fundamental would be substantially larger, as is the case for methyl halide complexes.20 It is interesting to note the av, fundamental trend of increasing displacement from the 3919-cm-' HF value with increasing halogen size for the HF- -XCN complexes and the similar trend with HF- -ClX' complexes (X' = CN, C1, and F). The product band absorbance also increased with the heavier halogens. These comparisons point to increased Lewis acid strength for the heavier halogens bonded to a common electronegative substituent (CN) and for chlorine bonded to more electronegative substituents. The Lewis acid character of the cyanide halogen and the basic character of the fluorine bonded to Hbinvite consideration of a cyclic structure for the 1:2 complex 3. New evidence suggests that the 1:2 HCN-(HF), complex is probably cyclic,21and the vQarv,b, vla,and vlb bands for 3 are similar to the bands observed for HCN- -(HF),. Furthermore, there is no evidence for Hb-F binding to site other than the Ha-F submolecule. It is quite likely that 3 is cyclic rather than open chain, but the infrared data are not conclusive on the structure of the 1:2 complex. Competing Effects. The halogen effect on the basicity of the nitrile lone pair, based on the v, mode positions, is not as pronounced as might be expected, due in part to the relatively large distance between the halogen substituent and the nitrile lone pair. In contrast, v, modes of the fluoramide and chloramide H F complexes were shifted substantially (3389 and 3311 cm-I, respectively) from the NH3--HF value (3041 cm-1).23,24 Fluorine
did reduce the basicity of the nitrile lone pair by the largest amount. On the other hand, chlorine had slightly less effect on the lone pair basicity than did bromine and iodine, which indicates that other factors in addition to simple inductive effects play important roles in the halogen and C N interaction. In addition to inductive effects, conjugation can affect the basicity of the bonding sites, as demonstrated by comparing the ClCN- -HF and (CN),- - H F complexes. Even though chlorine and the C N substituent have similar electronegativities, the basicity of the nitrile lone pair is much larger for ClCN than for (CN),. These competing effects, ?F conjugation and u induction, on the basicity of the nitrile substituent complicate the FCN, ClCN, BrCN, and ICN complex series since the induction effects are reduced and the ?F conjugation is increased as one proceeds down the halogen group.
Conclusions In summary, cyanogen chloride and H F in solid argon react to form a strong, well-defined 1:1 hydrogen-bonded complex of the form ClCN- -HF as well as an anti-hydrogen-bonded complex of the form HF- -ClCN. The matrix v, and vI modes are comparable to other cyanide-HF observations4-' for the hydrogenbonded product and provide spectral identification for HF bonded to the cyanide nitrogen with a linear structure. Interaction of HF with the base submolecule perturbed only the C N stretch, vi. The effect of the halogen substituents on the cyanide group is to withdrawn electron density and to reduce the basicity of the nitrile lone pair through inductive effects and perhaps conjugation. The anti-hydrogen-bonded complex and the analogous C12 and C1F species gave similar v, modes. In experiments with more concentrated samples, two additional HF reaction products were observed. These products were identified as the 1:2 C1CN-(HF)2 complex and as the 2:l (ClCN),-HF complex. Both complexes were characterized by considering the perturbation of the second submolecule upon the 1:l complex. Similar complexes were obtained for BrCN and ICN condensed with HF and DF. Finally, FCN and (CN), interact with HF to form only linear hydrogen-bonded cyanide complexes, demonstrating the lack of Lewis acid character for the fluorine and cyano substituents. Acknowledgment. We gratefully acknowledge financial support from National Science Foundation Grant C H E 85- 166 11, preliminary (CN), studies by R. T. Arlinghaus, and a cyanogen chloride sample from B. S. Ault. Registry No. Ar, 7440-37-1; HF, 7664-39-3;CICN, 506-77-4; BrCN, 506-68-3; ICN, 506-78-5; FCN, 1495-50-7. ~~
~
(23) Johnson, G. L.; Andrews, L. J . Am. Chem. Sac. 1982, 104, 3043. (24) Andrews, L.; Lascola, R. J . Am. Chem. Sac., in press.
