J. Phys. Chem. 1982,86,4375-4379
CF3N(0)OC(CH3),radical. We suggest that negative spin reaches the fluorines by a combination of a through-bond spin polarization from the nitroxide's oxygen atom (which is in the y position with respect to fluorine%)and a 1,3 p p interaction between the semioccupied orbital and the nonbonding p orbitals of the fluorine atoms.35 The (34)For some acyclic alkyl radicals which have negative spin on yhydrogen atom in appropriate conformations see: Ingold, K. U.; Walton, J. C. J. Am. Chem. SOC.1982,104,6167.For a theoretical study of this phenomenon, see: Ellinger, Y.; Rassat, A.; Subra, R.; Berthier, G. J. Am. Chem. SOC.1973,95,2372-3.Ellinger, Y.;Subra, R.; Levy, G.; Millie, P.; Berthier, G. J. Chem. Phys. 1975,62,10-29. (35)A 1,3p-p interaction might produce negative spin density at F either by the mechanism suggested in ref 33 or by the transmission of positive spin to the F 2p orbitals and a subsequent negative spin polarization of the F 1s electrons; see ref 36.
4375
magnitude37of the hfs produced by a 1,3 p-p interaction should be strongly dependent on the spatial separation between the fluorine atoms and the semioccupied orbital. The observed rather large and positive temperature coefficient of the F hfs (+2.8 mG/K) may be partly due to the increased amplitude of the CF3 group's vibrations (e.g., the umbrella motion) as the temperature rises. The conformation of a fluorine atom with respect to the semioccupied orbital will determine the final sign of its hfs.
Acknowledgment. We thank Dr. J. A. Howard for his advice and assistance with some of these experiments. (36)Karplus, M.;Fraenkel, G. K. J. Chem. Phys. 1961,35,1312-23. (37)The actual spin density at F is very small.
Complex between Water and Ammonia Bengt Nelander' and Lelf Nord Thermochemishy Laboratory, Chemical Center, Universit.v of Lund, S-220 07 Lund, Sweden (Received: May 4, 7982; I n Final Form: June 75, 1982)
The 1:l complex between water and ammonia has been studied by means of infrared spectroscopy in argon and nitrogen matrices. Water is H (or D) bonded to ammonia. HDO seems to be exclusively D bonded. The intramolecular vibrations of one of the molecules in the complex are remarkably sensitive to the isotopic composition of the other. The complex is strongly perturbed in nitrogen as compared to argon. In argon, a number of bands were observed in the 650-250-cm-' region. A few of these were also observed in nitrogen. An attempt is made to correlate the complex shifts of v2 of ammonia with the calculated heats of formation for a number of ammonia complexes.
Introduction The ammonia-water complex has been studied theoretically by several authors.1-8 Experimentally the crystalline ammonia hydrates have been investigated in considerable detail,%14but, apart from an observation of a water-induced band in the v2 region of a"onia,l5 nothing has been published on the binary complex. (1)P. A. Kollman and L. C. Allen, J. Am. Chem. SOC.,93,4991(1971). (2)G H.F. Diercksen, W. P. Kraemer, and W. von Niessen, Theor. Chim. Acta, 28,67 (1972). (3)L. Piela, Chem. Phys. Lett., 15, 199 (1972). (4)P. Kollman, J. McKelvey, A. Johansson, and S. Rothenberg, J. Am. Chem Soc., 97,955 (1975). (5)L. C.Allen, J. Am. Chem. Soc., 97,6921 (1975). (6)3. D. Dill, L. C. Allen, W. C. Topp, and J. A. Pople, J . Am. Chem. Soc., 97,7220 (1975). (7)H.Umeyama and K. Morokuma, J. Am. Chem. SOC.,99, 1316 (1977). (8)R. C.Kerns and L. C. Allen, J.Am. Chem. Soc., 100,6587(1978). (9)W. J. Siemons and D. H. Templeton, Acta Crystallogr., 7, 194 (1954). (10)I. Olovsson and D. H. Templeton, Acta Crystallogr., 12, 827 (1959). (11)J. E.Bertie and M. M. Morrison, J. Chem. Phys., 73,4832(1980). (12)G.Sill,U.Fink, and J. R. Ferraro, J. Chem. Phys., 74,997(1981). (13)J. E.Bertie and M. M. Morrison, J. Chem. Phys., 74,4361(1981). (14)C.Thornton, M. S. Khatkale, and J. P. Devlin, J. Chem. Phys., 76,5609 (1981). (15)L. Abouaf-Marguin, M. E. Jacox, and D. E. Milligan, J. Mol. Spectrosc., 67,34 (1977). 0022-3854/82/2088-4375$01.25/0
We therefore felt that a matrix isolation study of the ammonia-water complex would be of considerable interest in itself and also as a convergence point for our (hitherto parallel) investigations of water complexes'6-26and ammonia c ~ m p l e x e s . ~ ~ - ~ l
Experimental Section Two cryostats have been used in the work: an He cryostat which has been described earlier32and a cryostat (16)L. Fredin, Chem. Scr., 4,97 (1973). (17)L. Fredin and B. Nelander, J. Mol. Struct., 16,217 (1973). (18)L. Fredin, B. Nelander, and G. Ribbegird, Chem. Scr., 7, 11 (1975). (19)L. Fredin, B. Nelander, and G. Ribbegird, Chem. Phys. Lett., 36, 375 (19751. (20) L.'Fredin, B. Nelander, and G. Ribbegird, J. Chem. Phys., 66, 4065 (1977). (21)L. Fredin. B. Nelander. and G. Ribbegird. J. Chem. Phvs.. 66. 4073 (1977). (22)B. Nelander, Ber. Bunsenges. Phys. Chem., 82,61 (1978). (23)B. Nelander, J. Chem. Phys., 69,3870 (1978). (24)B. Nelander, J. Chem. Phys., 72,77 (1980). (25)L. Nord, J. Mol. Struct., submitted. (26)L. Nord, J. Mol. Struct., submitted. (27)G. Ribbegird, Chem. Phys. Lett., 25,333 (1974). (28)G.Ribbegird, Chem. Phys., 8, 185 (1975). (29)L.Fredin, B. Nelander, and G. Ribbegird, Chem. Phys., 12,153 (1976). (30)L. Fredin and B. Nelander, Chem. Phys., 15,473 (1976). (31)B. Nelander, J. Mol. Struct., 81,223 (1982).
0 1982 American Chemical Society
4376
Nelander and Nord
The Journal of Physical Chemistty, Vol. 86,No. 22, 1982
TABLE I: Assignments (cm-I) for the Ammonia-Water Complex in Solid Nitrogen at 20 K fundamentals
HOH.NH,
HOH.ND,
HOD.NH,
HOD.ND,
DOD.NH,
3414-3374
3 4 1 1-3367
2519.6 2503.8 1395.0
2514.5 2498.7 1394.5
2513.9 2501.6 1199 1197 2732.7 2726
DO D. N D
water LJ
9
1629 3697
3697
(3697)
2509.2 2495.9 1199 2731.2
ammonia v2
1068.3 1056 1046.7 439 4 1 4 (sh)
1068.4 1058 1047.2
819.3 811.3
820.0 812.2
1068.4 1058 1047.2
Low-Wavenumber Vibrations 418
820.0 812.2 331
TABLE 11: Assignments (cm-I) for the Ammonia-Water Complex in Solid Argon a t 11 K fundamentals
HOH.NH,
HOH.ND,
vi
3434.9
3428.3
V? v3
3702.2
HOD.NH,
HOD.ND,
2534.0 1391.4
2529.1 1392.1
DOD.NH,
DOD.ND,
water 2522.3
2517.7
2738.0
2736.5
1036.6
805.4
ammonia V?
(805
1035.4 638 420 402 295
i
3
*
10
i
2)
1036.6
805.4
Low-Wavenumber Vibrations 385 371 466 3 0 0 ?: 5 346 275 t 5
using an Air Products CS 208 refrigeration system with window and thermometer arrangements similar to the He cryostat. The deposition system has been described earlier.% The deposition rate was ca. 1.5 mmol/h except when otherwise stated. In most nitrogen matrix experimenta, 25-30 mmol was deposited at 20 K. In one case the deposition temperature was 17 K. Spectra were recorded at least at 10 or 11 K and at 20 K. In the 0-H stretching region spectra were of rather poor quality due to scattering. In a normal argon experiment, 7-9 mmol was deposited at 17 K. Thick (40-45 mmol) D20/ND3/Ar and ND3/Ar matrices were deposited at a somewhat higher rate at 11 K. Argon matrix spectra were recorded at 11 or 12 K. The concentrations of ammonia and water were varied in the approximate range 1/ 100-1/300. Ammonia (Matheson, lecture bottle, 99.95% pure) and ammonia-d3 (Roth, lecture bottle, >98 at. 5% D) were used after degassing. Water (H20)was doubly distilled, the last time in an all-glass apparatus, degassed, and distilled in a grease-free vacuum system. D 2 0 (99.5%) was degassed and used without further purification. An equilibrium mixture of approximately equal amounts of H20 and D,O was used to observe the HDO spectrum. Nitrogen and argon (L’Air Liquide 99.9995% ) were passed through a glass spiral immersed in N2(1)or 02(1), respectively. All infrared spectra were run on a Perkin-Elmer 180 instrument which was calibrated with standard gases.34 Wavenumbers given with a decimal are believed to be correct to 0.5 cm-’. Integer wavenumbers are of lower accuracy due to broad bands and/or overlap. When there (32)L.Fredin, K.Rosengren, and S. Sunner, Chem. Scr., 4,93(1973). (33)L.Fredin, Chem. Scr., 5, 193 (1974). (34)E.K.Plyler, A. Danti, L. R. Blaine, and E. D. Tidwell, J. Res. Natl. Bur. Stand. (US.), 64,29 (1960).
492 2 9 5 i 10
480 303
i
5
is reason to believe that accuracy is lower than f1.5 cm-’, a plus/minus value is given. Relative peak positions in each spectral region are believed to be correct to 0.1 cm-’. The spectrometer was connected to an LSI Alpha minicomputer (32K) with a Pertec disk and a Houston DP 1 plotter. Figures are reproduced directly from recorded spectra without calibration correction.
Assignment “Water”, “ammonia”,and the equivalent “aq” and “am“ will be used for all hydrogen-deuterium compositions of the compounds. A specific isotopic species will be indicated by its formula. In complexes of the type studied here, the intramolecular vibrations of the complex-forming molecules are only slightly shifted from the unperturbed positions. Therefore, in order to simplify the notation, the perturbed i-th fundamental of A in a complex with B will be denoted as vi(A*B). The i-th fundamental of water, which forms a hydrogen bond with ammonia, will be denoted as vi(HOH-am),otherwise as vi(H20.am). In the case of HDO complexes, H or D bonding will be indicated by writing H or D next to B, ui(DOH.am) or vi(HOD.am). In the concentration range used, the experimental technique gives predominantly monomer. 1:l complex bands appear at the lower concentrations used. Weak, strongly concentration-dependent absorptions of higher complexes are occasionally observed at concentrations approaching 1:lOO. Except as indicated below, the bands assigned to a water-ammonia complex were absent when only ammonia or water was present in the matrix, but present when both ammonia and water were present. For a given matrix material, the intensity ratios between bands assigned to a complex with given isotopic composition remained constant in all experiments where this species was present. The observed isotope shifts serve as an additional check on the assignments. Tables I and I1 summarize the bands
The Journal of Physical Chemistry, Vol. 86, No. 22, 1982 4377
Complex between Water and Ammonia
A
A 2k0
II
'2520
'2506
I
'2;BO
I I
I
Y 3'4Y0
31100
3360
3320
3M0
3WG
3400
F w e 1. u, region of water Kbondedto ammonia: (A) nibogen matrlx at 20 K, N,/ND, = 115, N,/H,O = 171, 26.7 mmol deposited; (B) argon matrix at 12 K, Ar/NH, = 122, Ar/H,O = 128, 8.7 mmol deposited.
assigned to ammonia-water complexes of different isotopic composition. Water-Ammonia in Nitrogen. 0-HStretching Region. When H 2 0 and NH3 are present in a nitrogen matrix, a rather strong and broad absorption band with maxima at 3414 and 3374 cm-' appears. With H 2 0 and ND,, corresponding maxima are found at 3411 and 3367 cm-', and two additional maxima at 3391 and 3376 cm-' are observed (see Figure 1A). The shape of the band is sensitive to the deposition conditions and the temperature at which the spectrum is recorded. In the presence of ammonia, the water dimer band at 3698.8 cm-' (ref 20) becomes significantly more intense relative to other dimer bands, and its maximum shifts to 3697 cm-'. We therefore assign the 3697-cm-' band to u3(HOH.NH3). We have not been able to find any isotope shift between u3(HOH.NH,) and u3(HOHeND3). When an equilibrium mixture of H 2 0 and D20 is substituted for H20, the 3697-cm-' band retains its intensity. We therefore believe that u3(HOD.NH3) coincides with u3(HOH.NH3) at 3697 cm-'. 0-D Stretching Region. When ammonia and D20 are simultaneously present in the matrix, an absorption band which is assigned to u,(DOD.am) arises in the 27332725-cm-' interval. In most experiments, the maximum of the band was found close to 2733 cm-', but in a few experiments the maximum shifted to 2726 cm-l. As in the 0-H stretching region, the bandshape is sensitive to the deposition conditions, which made it difficult to observe any isotopic shift. However, in experiments with identical conditions, there was a 1.5-cm-' red shift when ND, was substituted for NH3. The u1 band of complexed D20 has maxima at 2513.9 and 2501.6 cm-'. Between these two maxima, the absorption is rather high and in some experiments there is a third maximum at 2506 cm-'. The u1 absorption band of complexed HDO overlaps that of D20. From experiments with varying relative concentrations of HDO and D20, it seems clear that the shape of the band is similar to that of DzO,but with its maxima shifted toward higher wavenumbers. When ND3 is substituted for NH,, the 0-D stretching bands for both HDO and D20 shift ca. 5 cm-' toward lower wavenumbers (see Figure 2A). The relative intensities of the maxima of these bands vary reversibly with temperature in the 10-20 K interval and depend significantly upon the deposition conditions. Water Bending Region. u2(HOH.NH3)is hidden under u4(NH3),but uz(HOH.ND3)is observed as a somewhat
l
l
2560
,
i
l
I
25'40
l
l
l
!
252@
l
1
i
2500
Flgure 2. v, region of water Pbonded to ammonia: (A) Nitrogen matrix at 11 K. Upper: N2/ND3= 135, N,/aq = 166, H20/HDO/D20 = 1/2/1, 28.0 mmol deposited. Middle: N,/NH3 = 89, N,/aq = 113, HDO/D20 = 314, 27.3 mmol deposlted. Lower: N,/ND, = 83, N2/aq = 109, D,O >> HDO, 28.0 mmol deposited. (8) Argon matrix at 12 K. Upper: Ar/ND, = 119, Ar/aq = 92, H,O/HDO/D,O = 1/2/1, 9.0 mmol deposited. Middle: Ar/NH, = 124, Ar/aq = 127, H,0/HDO/D20 = 1/5/6, 8.1 mmol deposited. Lower: Ar/NH, = 123, Ar/aq = 91, H,O/HDO/D,O = 1/2/1, 7.8 mmol deposited.
ill-defined band at 1629 cm-'. u2(HOD-NH3)at 1395.0 cm-' is somewhat sharper than the corresponding HzO band. There seems to be a 0.5-m-' shift between u2(HOD.NH3)and u2(HOD.ND3)but, since these bands are somewhat sensitive to deposition conditions, the difference may not be significant. u2(DOD.NH3)is observed as a weak band with maxima at 1199 and 1197 cm-l. The relative intensities of the maxima are sensitive to deposition conditions. We are unable to observe any isotope shift between the NH3 and ND, complexes. u2 Region of Ammonia. vz(NH3.HOH)is observed as an intense band with at least three maxima, at 1068.3,1056, and 1046.7 cm-', the lowest being the most intense. There appears to be an isotope shift of 0.7 cm-' between uz-
4378
The Journal of Physical Chemistry, Vol. 86,No. 22, 7982
(NH,.HOH) and u2(NH3.DOD),with the D20 complex at the highest wavenumber. We have not been able to find any shift between u2(NH,.DOD) and u2(NH3.DOH),possibly because the half-widths of the two bands exceed their expected wavenumber difference. For u2(NH3.aq)we observe only two maxima; the third (and weakest) is probably hidden under the monomer band of ND2H. Also in these cases, there is an isotope shift between hydrogen-bonded and deuterium-bonded complexes, with the D-bonded complexes having the largest complex shift. Low-Wavenumber Region. The low-wavenumber region is difficult to study for two reasons. First, bands in this region tend to be broad and rather weak, and, second, the interference fringes resulting from internal reflections in the matrix are superimposed on the absorption spectra. Table I gives the bands that we believe can be assigned to ammonia-water complexes. Water-Ammonia in Argon. 0-H Stretching Region. A rather strong band a t 3702.2 cm-' is assigned to v3(HOH-NH,). With HDO present in the matrix, this band is obscured by the intense HDO monomer absorption. We do not observe u,(HOD.am). It may be hidden under the HDO dimer absorption a t 3693.6 ~ m - ' . , ~ Strong bands at 3434.9 (Figure 1B) and 3428.2 cm-' are assigned to u1(HOH.NH3)and u1(HOH.ND3),respectively. With H 2 0 and NH, there are three new bands, a t 3328 (medium), 3313 (weak and rather close to u1 of the ammonia dimer at 3310 cm-'), and 3283 (weak) cm-'. These bands will be discussed together with their 0-D stretching region counterparts. 0-D Stretching Region. Strong bands at 2738.0 and 2736.5 cm-' are assigned to v3(DOD.NH3)and v3(DOD. ND,) , respectively. Very strong bands a t 2534.0, 2529.1, 2522.3, and 2517.7 cm-' are assigned to vl(HOD.NH3), v1(HOD.ND3), u l (DOD.NH,), and v1(DOD-ND3),respectively (see Figure 2B). There are two bands from u3 of ND, in this region, a dimer band29 a t 2527.4 cm-I that introduces a slight asymmetry in the 2529.1-cm-' band and a polymer band at 2520 cm-I which is, however, too small to interfere with the 2517.7-cm-' complex band. There are several weak new bands in the low-wavenumber part of the 0-D stretching region. The most prominent appear a t 2463 cm-' with NH, and 2457 cm-' with ND,. A weak absorption at 2452 cm-' (NH,) does not seem to have any ND, counterpart. There are also complex, structured absorptions a t 2429 f 4 (NH,) and 2425 f 4 (ND,) cm-'. These bands obviously originate in u1 of water D-bonded to ammonia, but their positions imply a heavy perturbation compared with the intense complex bands. The band pattern in the low-wavenumber part of the 0-H stretching region is strikingly similar to what is found here. In both regions we tentatively assign these bands to water-ammonia complexes with a stoichiometry different from 1:l.
Water Bending Region. Sharp bands at 1391.4 and 1392.1 cm-' are assigned to u,(HOD.NH,) and u,(HOD. ND,), respectively. We do not observe any complex bands in the u2 regions of H 2 0 or D20. These regions are rather crowded and complex absorptions could easily be hidden under the water and/or ammonia bands. ul, u,, and u4 Regions of Ammonia. No features attributable to the ammonia-water complex are observed. As (35) G.P.Ayers and A. D. E. Pullin, Spectrochim. Acta, Part A, 32, 1629 (1976), and three more following papers. (36) J. A. Cugley and A. D. E. Pullin, Spectrochim. Acta, Part A , 29, 1665 (1973).
Nelander and Nord
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