4273
J . Phys. Chem. 1986.90, 4273-4282
ZR. Infrared spectroscopy confirms that the major product of the reaction of G a atoms with CO at cryogenic temperatures is Ga(C0)2. Formation of this molecule must involve the intermediacy of linear and not bent GaCO, a species that has tentatively been identified by I R but not EPR spectroscopy. co co Ga GaCO Ga(C0)2
- -
The relative intensities of the symmetric (Isrm) and antisymmetric (Zasym) CO stretching modes can be used to calculate the C-Ga-C angle 0 by using eq 226on the assumption that dipole moment derivatives are directed along the CO bonds. Isym/zasym
=i
cot2 (0/2)
(2)
Measurements of the areas under these bands at 77 K gave B = 120’ f 5. Furthermore measurements at higher temperatures suggested that B increased slightly as the temperature was increased. The frequencies of these two bands can be used to calculate the force constant kco and the interaction constant kcoco if we make the Cotton-Kraihanzel appr0ximation,2~which decouples the high-frequency ligand stretching modes from the other vibrations in the molecule. The secular equations used in this calculation are = PL(kC0 + kco.co)
(3)
and = P@CO - kco.co) (4) for the symmetric and antisymmetric stretching modes, respectively, where X = (5.8890 X 10-z)u2, P is the reciprocal of the reduced mass, viz., (16.00 f 12.01)/(16.00 X 12.01) = 0.14583 for 12C’60,and u is the frequency in cm-’. Substitution of the two frequencies into these equations gives k , = 15.67 and kcwo = 0.63 rndyn1.A. The CO force constants, unpaired spin populations, and C-M-C angles for A1(C0)2 and Ga(CO), in rare gas and hydrocarbon (26) Reference 16. p 697. (27) Cotton, F. A,; Kraihanzel, C. S. J . Am. Chem. SOC.1962, 84, 4432-4438. (28) Hinchcliffe, A. J.; Ogden, J. S.;Oswald, D. D. J . Chem. Soc., Chem. Commun. 1972, 338.
matrices are summarized in Table 11. The force constant km for Ga(C0)2 is measurably larger than the value for Al(C0)2, which indicates less charge transfer from the metal atom to the ligands for Ga. This is probably because the ionization potential of G a is slightly larger than that of A1 (6.00 and 5.98 eV, respectively). The unpaired spin populations on Ga and A1 for the dicarbonyls in argon are consistent with this difference. The C-M-C angle for Ga(C0)2 is IOo larger than it is for A1(C0)2 and in fact corresponds to that expected for sp2 hybridization. It is worth ending on a cautionary note. We, like Kasai and Jones,j have interpreted our results in terms of the “classical” molecular orbital model of donation of electrons from 5a C O orbitals into empty metal orbitals and back-donation of unpaired np, electrons into the 2n* C O orbitals. BagusZ9and Davenport30 have questioned the validity of this model. The former author’s theoretical calculations have shown that the 5a contribution to bonding is negligible and that bonding arises largely from delocalization of metal electrons into the 2n* orbitals. Davenport has demonstrated that the spectroscopic features of carbonyls, a downward shift in frequency and intensity enhancement, can be explained by an electrostatic model without recourse to a-7 donation and back-donation. The fact that Ga(CO), is a bent molecule seems to require a bonding accompanying hybridization and is difficult to explain in terms of Bagus and Davenport’s views since they would favor a linear molecule that reduces electrostatic repulsion.
-
Acknowledgment. C.A.H. thanks SERC for a studentship. B.M. acknowledges the financial help from SERC for the purchase of equipment, and J.A.H. and B.M. thank NATO for a collaborative research grant (No. 442182). We thank Drs. J. R. Morton and K. F. Preston for many helpful and stimulating discussions and Professor R. L. Belford for providing us with a copy of the computer program used to simulate anisotropic EPR spectra. Registry No. Ga(CO),, 95646-95-0; CO, 630-08-0; 69Ga(C0)2, 103323-08-6;Ga(CO), 103323-07-5;Ga, 7440-55-3; adamantane, 28123-2; cyclohexane, 110-82-7. (29) Bagus, P. S.; Nelin, C. J.; Bauschlicher, C. W., Jr. Phys. Reu. E Condens. Matter 1983, 28, 5423-5438. (30) Davenport, J. W. Chem. Phys. Lett. 1981, 77, 45-48.
FTIR Spectra of Methyl-Substituted Amlne-Hydrogen Fluoride Complexes in Solid Argon and Nitrogen Lester Andrews,* Steven R. Davis, and Gary L. Johnson Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: February 19, 1986)
Matrix infrared spectra of methyl-substituted amine complexes with hydrogen fluoride were assigned to 1:l and 1:2 complexes by concentration and sample-annealing studies. The 1:l complexes exhibited decreasing us (HF stretching) and increasing uI (HF librational) modes with increasing methyl substitution and perturbed -NH2 and -NH wagging and N - C stretching modes. The relative yield of 1:2 complexes increased substantially with methyl substitution. Two H-F stretching fundamentals were observed for (CH3)3N--(HF), along with extensive combination progressions in both hydrogen bond stretching modes. These modes characterize strong hydrogen bonds intermediate between those found in CH3CN--HF and (CHJ2C=O- -HF on the one hand and HF2- on the other and show that the inside proton is shared between N and the inside F and that the terminal H-F is elongated with the hydrogen still much closer to the terminal fluorine.
Introduction Hydrogen fluoride forms strong hydrogen-bonded complexes to strong bases, and substituted amines constitute an important group of bases capable of forming such strong complexes. The first member of this series, H3N- -HF, has been studied in detail
in solid matrices’ and observed in the gas phase by infrared2 and microwave SpeCtrOSCOPY? Matrix infrared spectra revealed a c3” (1) Johnson, G . L.; Andrews, L. J . A m . Chem. SOC.1982, 104, 3043. Andrews, L.J . Phys. Chem. 1984,88, 2940.
0022-3654/86/2090-4273$01 .50/0 0 1986 American Chemical Society
4274
The Journal of Physical Chemistry, Vol. 90, No. 18. 1986
Andrews et al.
us
0
I no
1600
iioo
1400
1500
ijoo
1100
Boo
Qoo
id00
'
WRVENUMBERS
Figure 2. Infrared spectrum of argon matrix containing hydrogen fluoride and monomethylamine prepared by d e p o s i t i n g Ar/R = 200/1 samples at 12 K. TABLE I: Infrared A b s o r p t i o ~( c d ) for Monomethylamine Complexes with HF and DF in Solid Arnon and Nitrogen
-
HF
3100
2900
2700
2500 2300 WAVENUMBERS
2100
1900
1700
Figure 1. Infrared spectra of samples prepared by d e p o s i t i n g hydrogen fluoride and methylamine mixtures a t 12 K using A r / H F = 150/1 and Ar/amine = 200/1 with 50-70 mmol of each sample deposited: (a) trimethylamine, (b) dimethylamine, (c) monomethylamine, and (d) ammonia with A r / H F = 300/1 and Ar/NH3 = 300/1. Atmospheric C02 has been removed from the spectra.
complex with a relatively strong hydrogen bond. The present matrix investigation was performed to characterize the methylation effect in a m i n e H F complexes. A recent theoretical study on this subject has found no significant effect on the hydrogen bond energy for methyl substitution: Extensive matrix isolation and theoretical studies have been done on the analogous HCl, HBr, and HI ~ystems.~-'~ In addition, hydrogen fluoride can readily form higher complexes, and the bonding characteristicsin the a m i n e - ( H q 2 series are of interest as well. The H3N--(HF)2species in solid argon gave evidence for a stronger primary hydrogen bond than the 1:l complex. These matrix studies bridge the gap between the gaseous 1:1 and condensed-phase studies of higher hydrogen fluoride c o r n p l e ~ e s . ' ~ - ' ~
(2) Thomas, R. K., unpublished results. (3) Clements, V. A.; Langridge-Smith, P. R. R.; Howard, B. J., to be published. (4) Szczesniak, M. M.; Hobza, P.; Latajka, 2.; Ratajczak, H.; Skowronek, K. J. Phys. Chem. 1984,88, 5923. (5) Ault, B. S.; Pimentel, G. C. J . Phys. Chem. 1973, 77, 1649. (6) Ault, B. S.; Steinbach, E.; Pimentel, G. C. J . Phys. Chem. 1975, 79, 615. (7) Barnes, A. J.; Beech, T. R.; Mielke, Z. J . Chem. SOC.,Faraday Trans. 2 1984,80, 455. ( 8 ) Barnes, A. J.; Kuzniarski, J. N. S.; Mielke, Z. J. Chem. SOC.,Faraday Trans. 2 1984,80,465. (9) Schriver, L.; Schriver, A.; Perchard, J. P. J. Am. Chem. Soc. 1983,105, 3843. (IO) Clementi, E. J. Chem Phys. 1967,46, 3851; 47, 2323. Clementi, E.; Gayles, J. N. J. Chem. Phys. 1967, 47, 3837. (11) Raffenetti, R. C.; Phillips, D. H. J . Chem. Phys. 1979, 71, 213. (12) Latajka, Z.; Sakai, S.; Morokuma, K. Chem. Phys. Lett. 1984, 110, 464. (13) Latajka, Z.; Scheiner, S . J. Chem. Phys. 1985, 82, 4131. (14) Harmon, K. M.; Gennick, J. J. Mol. Struct. 1977, 38, 97. (15) Harmon, K. M.; Lovelace, R. P. J. Phys. Chem. 1982, 86, 900. (16) Mootz, D.; Pool, W. 2. Naturforsch. 1984, 396, 290. (17) Mootz, D.; Boenigk, D., to be published.
2816 1960 1809 1706 1598, 1595 1524 1418 1315 1174 1036 1030 934 869 2633 1945 1602 1179 1110 96 1 904
DF
assignt'
Argon 2138
"S
A
1317
"sa
1596, 1592 1206
A (1 624) A A
"4
A 1174 74 1 1034 968, 933c 683
(1141)
V,
",(IDb
(1053) (797) "9 "I(I)
Nitrogen 1962 1404 1602 1179 801 974 712
"a
"sa
(1643) (1144) "1
(11)
(817) "I
"4
"7
"9
(1)
'All are 1:l complexes except the vP and A bands. Matrix values for M M A fundamentals are given parenthetically, and assignments are from ref 19. (11) motion is in the C-NH2 symmetry plane and the v,( I)motion is perpendicular to the symmetry plane. Fermi resonance doublet probably involving usc and ~ ~ ( 1 1 ) Y,.
+
ExperimeaW Section The vacuum and cryogenic apparatus have been described in earlier papers.**'* Spectra were recorded with a Nicolet 7199 FTIR spectrometer in the 4000-400-cm-' range at 1-cm-' resolution. Reagent gases, obtained in lecture bottles from Matheson, were condensed and degassed, and a middle fraction was used. Deuterated reagents CH3ND2and (CD3)3N were obtained from MSD Isotopes; the manifold was deuterated with D 2 0 , and CH3ND2was exchanged with the system before use. Triethylamine (Matheson, Coleman and Bell) and piperidine (Fisher) were frozen and degassed. Hydrogen fluoride (Matheson) was degassed at 77 and then at 180 K,and deuterium fluoride was synthesized by reacting low-pressure elements in a sample can and manifold used only for DF, which enabled DF enrichments above 90% t o be attained in these experiments. Argon solutions of hydrogen fluoride diluted to mole ratios ranging from loo/ 1 to 600/1 and amine dilutions of 200/ 1 and 300/1 were used in this study. Several experiments also employed nitrogen as the matrix material. The reagent solutions were simultaneously expanded through twin jets o n t o a 10-12 K CsI window at 2-3 mmol/h each for 16-20 h in slow experiments and 5-8 mmol/h each for 3-5 h in fast experiments for each meth(18) Johnson, G. L.; Andrews, L. J . Am. Chem. SOC.1980, 102, 5736.
Spectra of Amine-Hydrogen Fluoride Complexes yl-substituted amine. The spectrum was recorded, the window was warmed to 18-22 K for 10-15 min and then recooled to 10-12 K, and another spectrum was recorded; this cycle was repeated in the 28-32 K range for selected samples. Amineargon matrices were also studied without hydrogen fluoride to identify the new amine-HF complex bands.
Results Hydrogen fluoride and substituted amine codeposition experiments will be described in turn for different amines. Methylamine. The major product with monomethylamine (MMA, CH,NH2) and HF gave a strong absorption at 2816 cm-' on the side of a weaker 2820-cm-' MMA band that is compared in Figure 1 with the major product absorptions (labeled v,) from similar ammonia, dimethylamine, and trimethylamine experiments. In addition, the weaker absorption at 1809 cm-' (labeled vSa) exhibited a greater increase on sample annealing to allow further association of HF relative to the v, band, which indicates that the 1809-cm-I absorption is due to a species containing more H F submolecules than the 2816-cm-' band. The lower region of this spectrum is illustrated in Figure 2, and the product absorptions are collected in Table I. Precursor (or base) absorptions are labeled vi, following the assignments of an earlier study,19 and product complex absorptions associated with that precursor mode are marked u:. Additional sharp bands at 1036 and 869 cm-I, labeled vI,are hydrogen fluoride motions as attested by their large displacement to 741 and 683 cm-l, respectively, with deuterium fluoride. The us and v, absorptions were found at 2138 cm-l with a 21 27 cm-' shoulder, and 1317 cm-I, respectively, with DF, and small D F shifts were observed for other bands as given in Table I. Matrices containing M M A alone were examined and bands ~ of are in f l cm-' agreement with reported v a l ~ e s . 'Annealing M M A samples caused significant diffusion and aggregation of MMA itself as attested by growth of strong broad bands at 3385, 3280, and 3180 cm-', which are appropriate for hydrogen-bonded N-H stretching vibrations. Likewise, in the CH3ND2experiments to be described below, these bands were replaced by strong new absorptions at 2525, 2460, 2383, and 2348 cm-I, which are appropriate for the N-D counterparts. In addition, further new product absorptions that require both reagents and increase markedly on sample annealing are labeled A (for aggregate) in the tables. Methylamine was also d e p o s i t e d with HF in excess nitrogen to examine the effect of a different medium on the product complexes. The strong v, band was markedly red-shifted and the v, band was blue-shifted, giving the major component in Table I and a number of satellites. Enhanced stabilization of the 1:l complex by solid nitrogen blue-shifted both vI modes such that the upper vl mode was now clearly separated from precursor bands. A D F experiment was also performed and counterpart bands are given in Table I. Monomethylamine-d2 was also examined in several experiments using solid argon: based on the relative intensities of the resolved H3C-NH2, -NHD, and -ND2 bond stretching modes (v8), the sample in the matrix was greater than 85% CH3ND2,with most of the balance as CH3NHD. The major product band (v,) became a 2808-, 2793-cm-' doublet and the minor (u,) band blue-shifted to 18 18 cm-I, and other sharp product absorptions are given in Table 11. Deuterium fluoride substitution shifted the strong doublet to 2128, 2119 cm-' and the v,, band to 1307 cm-' and slightly displaced some of the uf bands, most notably vgC, which was observed at 732 cm-' with HF and at 716 cm-' with DF. Two base submolecule modes were observed for CH3NHD and each isotopic acid, which are given in Table 11. Dimethylamine. The strongest product band with dimethylamine (DMA, (CH,),NH) and HF appeared at 2671 cm-' (v,), but the 1822-an-' band group (u,) in Figure 1 runs a close second in intensity. The lower region (Figure 3) reveals product bands (19) Purnell, C. J.; Barnes, A. J.; Suzuki, S.;Ball, D. F.;Orville-Thomas, W. J. Chem. Phys. 1976, 12, 7 7 .
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4215 TABLE 11: Infrared Absorptions (cm-') for Monomethylamine-d, Complexes with HF and DF in Solid Argon HF
DF
2807, 2793 1960 1818 1737 1503 1418 1362 1303 1198 1146.5 lOOlb 985 732c 788
2128, 2119 1397 1307
assignt' US
A yea
A A A A A (1203) ~q (1131) UT (1 006) Y S
1173
1198 1145.6 991 778 716 672
VI
(11)
(638) VI
v9
(1)
'All are 1:l complexes except urn and A bands. Matrix values for MMA-d2 fundamentals given parenthetically; assignments are from ref 19. bFor the CH,NHD base, this line becomes 1030, 1029, (1042). CFor the CH3NHD base, this line becomes 823, 816, (701).
0
cn
I
a
m
0
P
IC
0
w(D u
z a I-
C
I-
-a In v) z
P
U
e
P
I-0
x"
a
m
a
N
P
0,
z/c
P
P
0
1200
li00
rboo
do0
Boo
$00
WAVENUMBERS
Figure 3. Infrared spectrum of argon matrix containing hydrogen fluoride and dimethylamine prepared by codepositing Ar/R = 200/1 samples at 12 K.
(labeled vI) that show large displacements with DF and new bands (marked uf) that show small or no displacement with D F and are associated with a particular precursor mode. Sample annealing reduced HF and increased (HF), absorptions,2o and also reduced DMA and increased the v, and associated absorptions by 50% and the v, and associated bands by 100%. Table I11 lists the product bands and designates the v, group as 1:l and the us, group as 1:2 complexes. The strong v, and v, bands exhibited large D F shifts, and these bands are listed in the table in their grouping by annealing behavior. The sharp bands assigned to base submolecule (20) Andrews, L.;Johnson, G. L. J . Phys. Chem. 1984,88, 425.
4276 The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
Andrews et al.
TABLE III: Infrared Absorptions (cm-') for Dimethylamine Complexes with HF and DF in Solid Argon and Nitrogen HF 267 1 208 1 191 1 1851 1822 1229 1190.9 1082 1028 966 918 912 819.3 2574' 1922' 1197' 1028' 950' 911h 841h
DF 2012 1585 1408 1346 954 1192.9 894 1026 740 sh 696 912 822.0
species 1:l 1:2 1 :2 1:2 1:2 1:2 1:l 1:2 1:l 1:l 1:l 1:l 1:l 1:l 1 :2 1:1 1:1 1:l 1:l 1:l
assign" us
+ uE + usa
+
site "sa '?a
(1 151) CH, rock Vla
(1021) asym C-N VI VI
(929) sym C-N (735) N-H wag "S
"sa
(1145) (1021) VI
(930) (749)
Matrix values for DMA fundamentals are given parenthetically; assignments are from ref 21. 'Nitrogen matrix values.
modes2' in the 1:l complex exhibited shoulders appropriate for this motion in the 1:2 complex. One experiment was done with DMA and H F in solid nitrogen; again the v,, band was considerably stronger than the v, band, which are listed in Table 111. Trimethylamine. This symmetrical base (TMA, (CH3),N) was thoroughly examined with a 6-fold range of HF concentrations to distinguish between lower and higher order complexes. Notice in Figure l a that the v,, band at 1870 cm-' is stronger than v, at 2589 cm-' and that there are combination bands (marked with arrows) associated with vSa. Sequential annealing experiments, such as that illustrated in Figure 4, using A r / H F = 600/1, show a more pronounced growth for the v, and v,b and other associated bands (relative intensities 1.0/1.6/3.0 for three spectra in Figure 4) than us and associated bands (relative intensities 1.0/1.3/1.5 for three spectra in Figure 4). In a parallel Ar/HF = 200/1 study, the product bands were intially 3-fold stronger than in Figure 4, and two annealing cycles caused a 3-fold increase in v, and associated bands identified as 1:l in Table IV and an 8-fold-increase in v, vsb, and associated bands identified as 1:2 in the table. In addition, the band marked A in Figure 4 using fast deposition was absent with slow deposition (see Figure I), and this band increased still more dramatically on annealing (relative intensities 1/3/10 in Figure 4). Several experiments were performed with D F by using slow and fast deposition rates and annealing to identify D F counterparts of the many product bands due to the three above characterized species; these bands are also given in Table IV. Figure 5 compares H F and DF experiments with TMA; notice the large DF shifts for the vs, vsa,and vsb bands and similarity in spacing in the combination progressions. The band at 1847 cm-I appears to be due to a mixed HF/DF species based on several DF experiments with different levels of HF. A set of parallel experiments was performed with (CD3)3N using H F and DF; the spectrum of the base was similar to that reported earlier except for several weak bands.22 Displacements were observed in the product bands, including those due to acid submolecule modes, as are detailed in Table V. Figure 6 shows the 700-1 300-cm-' region of the spectrum from an HF experiment; the d9 base absorptions allow the librational modes for the 1:l and 1.2 species to be observed, whereas the h9 base absorptions obscured the lower two of these product absorptions. Figure 7 shows the 600-3000 cm-' region from a mixed HF/DF experiment and the similarity in combination progressions with the h9 precursor. Far-infrared experiments were done for each isotopic acid (21) Garner, G.; Wolff, H. Spectrochim. Acta, Parr A 1973, 2 9 4 129. (22) Goldfarb, T. D.; Khare, B. N. J . Chew. Phys. 1967, 46, 3379
I
x
u,&
TABLE I V Infrared Absorptions (cm-I) for Trimethylamine Complexes with HF and DF in Solid Argon at 10 K HF 3267 3178 2798 masked 2687 2654 2601 2589 2548 2464 2290 2218 2141 2111 2048 1898 1870 1215 1115 (1030)c 1020 masked 8 15.3 453
DF
assimt"
2419 2384 2798 2176 2054 2029
A A A (C-H str) us u, (1:l)
1959 1928
b 1822 1766 1586 1416 1397 1215 753 725 1026 64 1 816.3 447
+
usb usb
+ "Ba + uBa
site us (1:l) usb (1:2) "sa
usa
usa
A
+ "oa + "ob
+ vw. + 2''ob + ucb + "b + "sb + "Eb +
uob
(1:2) (1269) ~
JJB.
2 (0 1 ~ 1 )
ha
(1:l) (1039) u,
y19
"lb
(822) (368)
ug V,
(1:l) (1:2)
"All are 1:2 complexes except where noted 1:l or A. Matrix values for TMA fundamentals are given parenthetically; assignments are from ref 22. 'Sharp band at 1847 cm-' for (1:2) complex is probably due to v,,(hF) in TMA-HF--DF mixed complex, where usb(DF) is masked by the strong 1959-m-' band. 'Shoulder on red side of strong 1039cm-I precursor band.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4211
Spectra of Amine-Hydrogen Fluoride Complexes
W
u
Z U I-
+ l-l
vr
Z U
[r
I-
2100 1800 1500 1200 900 WAVENUMBERS 200/1 samples at 12 K: (a) HF, Figure 5. Infrared spectra of isotopic hydrogen fluoride samples (Ar/HF = 150/1) deposited with Ar/(CH&N (b) DF with about 15% HF. 3000
2700
2900
TABLE V Infrared Absorptions (cm-I) for Trimethylartline-d9 Complexes with HF and DF in Solid Argon
HF 3267 3178 2778 2624 2603 2568 2550 2515 2394 2064 masked 2019 1946 1926 1882 1845 1205 1132 1112 1071 1045 1042 1009 824 730.4 373
DF 2419 2384 masked 1910 1989
assignt' A A
masked
+ yo "sb + u@b Ysb + u@b
2563 2492 2393 2333 2239 2132 208 1 1909 1810 1330 1218 1130 1055 1025 933 873 836 462
VI
US
"sb
A Yy
2064 1765 1581 1448 1400 1204 1132 745 1070 1045 789 1011 642 729.0 370
TABLE VI: Infrared Absorptions (cm-I) for Trimethylamine Complexes with HF and DF in Solid Nitrbgen at 10 K HF DF TMA
+ urn + U,b
(1:2)
A (C-D str) v, + urn v, + uvb %n
+ UBI
V.I
+ u@a
A
(1:2) (1224) ~ 2 0(1:l) (1152) VIS ( 1 3 (1065) ( 1049) UI (1:l) ( 1003) ulb (1:2) (737) ug (1:1) (309) ~7 (1:2) u*
'All are 1:2 complexes except where noted 1:l or A. Matrix values for TMA-d, fundamentals are given parenthetically; assignments are from ref 22. to observe new bands in the 300-cm-l region. Finally, experiments were performed for (CH3)3Nand HF and D F in solid nitrogen and the vg region for the HF experiment is shown in Figure 8. Notice first that the Y, band is considerably stronger than the us band; since N, has a lower freezing point than
2132 2058 1924 1908 1973
vs
+ vo
vsb vsb
+
(l:2)
us (1:l)
1610 1420 1384 1226 1218 718 742 1027 738 684 648 455
Ar, the nitrogen matrix freezes more slowly, allowing more diffusion and association of HF. Notice second that v, is red-shifted 97 cm-' from the argon matrix value, and that v,, and vSb are blue-shijted by 39 and 15 cm-', respectively. Notice third the pronounced growth of these sharp bands on annealing the matrix to 26 K and the appearance of a broad A band at 1810 cm-l. In the lower region, and red-shift of us is accompanied by a blue-shift in Y, for the 1:l species to 1055 cm-I, where this sharp band is completely resolved from the 1037-cm-l precursor band. Observations from the analogous D F experiment are given in Table VI. Larger Substituted Amines. Similar argon matrix experiments were done for piperidine (P,(CH2)5NH)and the ethylamine series, and the prominent acid submolecule modes for these complexes
4278
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
J
;"
T s 0
N -
P
P
Andrews et al.
P
o
0
IO
-I
2700
&so
WAVENUMBERS
Figure 6. Infrared spectra in the 700-1300-~m-~region for trimethylamine-d9 deposited with hydrogen fluoride (Ar/R = 200/1): (a) spectrum of sample deposited at 10 K, (b) spectrum after warming to 35 K for 10 min and recooling to 10 K.
2300
&so
zioo
1950
1400
WVENUMBERS
Figure 8. Infrared spectra in the 1800-2700-~m-~region for trimethylamine and hydrogen fluoride samples in nitrogen (N2/R = 200/1): (a) spectrum after codeposition at 10 K, (b) spectrum after warming to 26 K for 10 min and recooling to 10 K. TABLE VII: Infrared Absorptions (cm-') for Piperidine and Mono-, Di-, and Triethylamine Complexes with HF and DF in Solid Argon
HF 2630 2608 2260 2062 1857 1094 1085 masked 1108 1012 Figure 7. Infrared spectrum in 600-3000-~m-~region for trimethylamine-d9codeposited with HF and DF samples (Ar/R = 200/1) at 10
K. are collected in Table VII. In the piperidine study the us and u, bands were of comparable intensity. The monoethylamine (MEA) experiments with HF revealed v, as a strong shoulder at 2750 cm-' on a 2779-cm-I MEA absorption, the major v,, peak a t 1827 cm-I with weak sharp u1 modes at 1071 and 868 cm-I, and a perturbed -NH2 wagging mode at 916 cm-I; D F counterparts are given in the table. Diethylamine (DEA) and HF gave a Y, band a t 2647 cm-I with a shoulder at 2615 cm-' and a comparable u, band at 1855 cm-I with combinations at 1962 and 2092 cm-'; the strongest new band in the lower region at 836 cm-I is probably associated with the -NH wagging mode at 739 cm-'. Triethylamine (TEA) was complexed in several studies. Experiments with HF gave an extremely strong us, band at 1889 cm-]
2750 1827 1589 1071
916 868 2647 2615 1962 1855 836
DF Piperidine 976 945
945 875 103 012
assignt" Y , (1:l) site
site "lb
(1118) (1037)
Monoethylamine 2097 us (1:l) 1331 1589
masked 922
masked Diethylamine
vsa
1624 -NH2 bend
"I
(11) (1:1)
(778) -NH2 wag Vi
(1) (]:I)
us (1:l)
site (1% (739) -NH
"sa
wag
Triethylamine 2527 1937 us ( 1 : l ) 1889 1437 ua (1:2) "All are 1:l complexes except where noted 1:2 or A; precursor bands from this work are given in parentheses.
Spectra of Amine-Hydrogen Fluoride Complexes ( A = 0.7) and a weaker v, band a t 2527 cm-' ( A = 0.08) with a 2469-cm-' shoulder, and the DF experiment gave a weak v, band at 1939 cm-' and a strong v,, band at 1437 cm-I.
Discussion The product absorptions require both reagents, and these bands can be separated into three general groups for each amine based on relative band intensities in deposition experiments performed with stepwise concentration changes and with argon and nitrogen matrices and on sample annealing. The sequential annealing experiments allow controlled diffusion and further aggregation of reagent submolecules, and the relative stoichiometries of the product complexes can be determined; careful annealing experiments were necessary to grow the product bands. In ammonia studies, the strongest product band in the H-F stretching region (3041 cm-I) correlated with the gas-phase spectrum (3215 cm-I) and was assigned to the H3N- -HF complex.' Accordingly, the strongest product band in the monomethylamine-HF experiments (v, at 28 16 cm-I) is assigned to the 1:1 amine- -HF complex. Bands that show the next most pronounced growth on annealing (inF)~ A cluding us,) are assigned to the 1:2 a m i ~ ~ e - - ( H complex. final group of bands (A) are very weak in the initial deposit but grow most markedly on sample annealing; these are associated with higher aggregates, which may be 2:l in the case of the smaller monomethylamine base but are probably 1:3 in the case of the larger trimethylamine base. The vibrational motions in amine-hydrogen fluoride complexes are characterized as follows in the tables. The symbol v, denotes the H-F stretching fundamental vibration in 1:l amine- - H F complexes 1, which are characterized by v,(HF)/v,(DF) isotopic
1
ratios in the 1.32 f 0.01 range for these studies. The vl label identifies the H-F librational motion arising from the two rotational degrees of freedom of the H-F monomer; this motion is doubly degenerate for the ammonia and trimethylamine bases. Such librational modes are characterized by vI(HF)/vl(DF) isotopic ratios ranging from 1.27 to 1.42 in these studies. Base submolecule modes in the complexes are perturbed from unresolvable to large (up to 157 an-')amounts from the base molecule values, and these bands are assigned accordingly. The 1:2 amine- -(HF), complexes 2 contain two H-F submolecules, and R,
A-
-
R
Hb
\
'F
2
these are noted a (first, inside) and b (second, outside). The symbol v,, denotes the H-F stretching mode for the H,F submolecule bonded to the base and v,b denotes the H-F stretching mode for the HbF submolecule bonded to the fluorine in H,F; these modes are Characterized by H F / D F isotopic ratios ranging from 1.32 to 1.39. Librational modes for the 1:2 complexes are given the vla and vlb designations. Specific vibrational characteristics of the complexes for each amine will be described next. Methylamine. The CH3HzN--HF complex v, fundamental (2816 cm-') is displaced from isolated HF (3919 cm-') even more than the H3N--HFcomplex (3041 cm-') so even higher v1 modes and larger perturbations in base submolecule modes may be expected here. The symmetrical NH3 base supported a degenerate v I mode at 916 cm-' in the HF complex,' but the reduced symmetry of the CH3NH2base is expected to lift this degeneracy, and two vI modes were observed for the CH3HzN--HF complex at 1036 and 869 cm-' in solid argon and at 1 1 10 and 904 cm-' in solid nitrogen. This rather large separation between the two
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4279 librational mode components is a consequence of asymmetry in the base owing to the single methyl substituent and interaction between the HF librational mode parallel to the symmetry plane and the symmetrical vSc (C-N str) and v9c ( N H 2 wag)lg base submolecule modes. The latter base submolecule modes are forced lower by interaction with the HF libration, but in the DF complex, the D F libration shifts below the base submolecule modes. The interaction is thereby reduced and of the opposite direction, and the base submolecule modes increase slightly in the D F complex (Table I). A similar effect was observed for the -NH2 wagging modes in the C2H5NH2complexes with H F and DF. The symmetric deformation base submolecule mode in N H 3 is perturbed most in the complex' (126 cm-' above the average of inversion-doubled v2 valuesz3),and the symmetric wagging mode for CH3NH2is blue-shifted a slightly greater amount (1 37 cm-I) in the H F complex. This blue-shift is due to repulsions between the amine and acid hydrogens, which increase for this vibrational coordinate. On the other hand, the symmetric -NH2 bending mode (v4) is red-shifted 26 cm-' in the HF complex, which may reflect a decrease in lone pair-bonded pair repulsions owing to polarization of the lone pair by HF. The small additional red-shift in vqCfor the D F complex (2 cm-') is due to a small interaction with v,(DF). The N-H2 stretching modes in the complexes were not resolved from the base spectrum. In CH3ND2,the change in base submolecule vibrations affects the acid submolecule modes to a small but measurable extent. First, the v,(DF) mode is red-shifted 10 cm-', and the vI (HF) modes are red-shifted 51 and 81 cm-', respectively, in the CH3ND2 complexes. These shifts are due to alterations in the interactions between base and acid submolecule modes. Interaction between vI (DFJI) at 778 cm-' forces ugC(NDz wag) lower to 716 cm-I, as compared to the 732-cm-l v9c value for the H F complex with vI(HF,J) higher at 985 cm-l. Finally, the symmetric -ND2 bending mode is red-shifted only 5 cm-' in the complex, in contrast to the 26-cm-I displacement for the -NH2 motion. The 1:2 complex with methylamine was characterized only by the v, band at 1809 cm-' with a site-split component at 1824 cm-'. With dimethylamine and trimethylamine, however, the higher relative yield of the 1:2 complexes provides more information on these higher species. Dimethylamine. The v, fundamental in the (CH3)?HN--HF complex is displaced still further (to 2671 cm-I) below the NH, complex value and two vl modes are observed above the NH3 complex value. The N-H wag is blue-shifted substantially in the complex, and the H F / D F effect on this base submolecule mode in the complex is due to interaction with the librational modes of appropriate symmetry. Both N-C stretching modes exhibit 1:l complex values with shoulders for the 1:2 complex. The strong v,, modes in the 1:2 complex exhibited two combination bands, which probably involve bending and stretching of the Hb-F hydrogen bond. Such combination progressions were considerably stronger in the TMA- -(HF), complex. Trimethylamine. The spectrum of TMA-hydrogen fluoride samples is complicated by the large yields of both 1:l and 1:2 complexes and extensive base absorption, which masks several important product bands. Nevertheless, an extensive series of experiments with different concentrations, different annealing temperatures, nitrogen matrices, and the TMA-d9 precursor enabled the spectra of two major products to be identified. The 1:l TMA- -HF complex was characterized by four bands: v,, vI and 2 perturbed base C-N stretching modes, the latter showing redshifts from the base itself.2z The vl mode is resolved from the precursor in the d9 complex (Figure 6 ) , and it exhibits a rather large H F / D F ratio owing to repulsion between the methyl and acid hydrogens, which tend to sharpen the librational potential well. There appears to be a small interaction between the v I and vgc modes for both hg and dg complexes based on small differences in the vgc modes (Tables IV and V) with H F and D F in spite of the fact that the vl motion is E in CJL: symmetry and the vg mode (23) Cugley, J. A.; Pullin, A. D.E.Spectrochim. Acta, Part A 1973, 29A, 1655.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
4280
TABLE VI11 usa US8
TMA-ha + HF
interval;
TMA-hg + DF
intervals
TMA-$ intervals TMA-d, intervals
+ HF + DF
+
Yrb
Ysa
+
vsa
+
usa + ura Yob
+
una
2Ynb
1870 2048 2218 2290 2465 (178) (178 + 170) (420) (420 + 175) 1397 1586 1766 1822 (189) (189 + 180) (425) 1845 2019 masked masked 2394 (174) (375 + 174) 1400 1581 1765 (181) (365)
TABLE IX TMA-ha + HF
intervals TMA-h9 intervals
+ DF
b
2548 1929
usb
+
+ vob + Y@b
vBa
2654, 2687 (106, 139) 2029, 2054 (100, 125) Ysh
21 1 1 . 2141 (63, 93)
Yeh
+
YRh
+ HF
2550
2603, 2654 (53, 74)
TMA-d9 + HF
Ysa
1845
usa + VOa 1926, 1946
TMA-d, intervals
(81, 101)
is A,; this interaction probably arises from anharmonic coupling between the vI and V g c motions. The weak 2176-cm-l band in Figure 5b appears to track with the strong v,(DF) band at 1959 cm-', and this 217-cm-l interval is appropriate for the low-frequency hydrogen-bond stretching mode us. The region above u,(HF) is masked by methyl C-H modes; however, in TMA-d9 experiments with HF a similar 2778-cm-' product band appeared to track with the strong v,(HF) band at 2568 cm-I. This 210-cm-' interval is also appropriate for the u, mode. Finally, 10-50-cm-' displacements in the us and uI acid submolecule modes on d9 substitution in the base demonstrate that a definite acid-base complex is formed in these experiments. The 1:2 TMA--(HF)2 complex exhibits a much more extensive spectrum with substantial d9 substitution effects as well. The key absorption for this species is the strong, sharp 1870-cm-I band (vsa, Figures 1, 5, and 6), and the extensive combination bands involving the 1870-cm-' fundamental are the most interesting part of the spectrum. Although the strong v,, band exhibited major D F and minor d9 substitution effects and is clearly due to a stretching mode involving H,F, the two combination intervals are relatively insensitive to isotopic substitution as illustrated in Table VIII. An additional strong band in each experiment was about ' / 6 of the intensity of v,, and did not fit any of the combination progressions. This band is assigned to the stretching mode of HbF, usb, in the 1 :2 complex. Additional sharp doublets observed in combination with the u, and v,b modes are listed in Table IX; these bands show small isotopic effects. In addition to the uI motions of the Ha-F and Hb-F submolecules, there are two other low-frequency modes involving the hydrogen bond that could readily combine with the strong v, and uSbmodes, namely the stretching mode of the hydrogen bond itself ~ ~ the spectra of bifluoride species)25and the (above noted v , ) (see degenerate bending mode involving primarly fluorine (denoted ug). These motions have been determined for the strong complex CH3CN- -HF in the gas phase from both microwave (vo = 181 f 20 cm-I, up = 45 f 15 cm-1)z6and infrared measurements (vu = 168 3 cm-', vB = 40 14 cm-I for the H F complex and u, = 160 20 cm-I, up = 39 14 cm-' for the DF complex).27 On the basis of substantially greater displacement in the us modes for
*
* *
(24) Pimentel, G. C.; McClellan, A. L. The Hydrogen Bond; W. H . Freeman: San Francisco, 1960. (25) Dawson, P.; Hargreave, M. M; Wilkinson, G.R. Spectrochim. Acta, Part A 1975, 3JA, 1055. (26) Bevan, J. W.; Legon, A. C.;Millen, D. J.; Rogers, S. C. Proc. R. SOC. London, A 1980, A370, 239. ( 2 7 ) Thomas, R. K. Proc. R. Soc. London, A 1971, A323. 133.
Andrews et al. TMA--(HF)z as compared to v, for the acetonitrile complex (3627 ~ m - ' ) , ~the ' present complex is substantially stronger and higher us and vB modes are expected. However, the hydrogen bonds in TMA- -(HF), are not as strong as in (HFz-) based on us, so the u, modes should be lower than the HF2- value (vl = U, = 600 ~ m - ' ) . ' ~Accordingly, on the basis of isotopic shifts, the intervals in Table VI11 are readily assigned to the two vu modes in the TMA- -(HF), complex, and the doublet intervals in Table IX are assigned to the vB modes of each HF submolecule with the degeneracy lifted in the asymmetric 1:2 complex. The vu vibrational intervals in Table VI11 are the first measured for a 1:2 complex and are of considerable value in relating this complex in solid argon to other HF complexes in different media. The vs and vu bands demonstrate that the first hydrogen bond (a) is stronger than the second (b) and that both are stronger than in CH3CN- - H F but weaker than in HFz-. The isotopic studies show that d9 substitution has little effect on the hydrogen bond, as expected, but that D F substitution leads to a slightly stronger hydrogen bond and verifies (from the small blue-shift) that these are in fact u, modes. Accurate Raman spectra of K+HFz- and K+DF2-show a +1 cm-' blue deuterium shift for the v , mode.z5 The observation of intense combination progressions in una and uub modes again follows the exampleZ5set by HFz- and reveals considerable anharmonic coupling between the v, and vu motions. Even with the first sum band fully absorbing, no difference band could be detected for the 1:2 complex. The vB intervals in Table IX are expected to exceed the lower 45 f 15 cm-' value for the weaker hydrogen bond in CH,CN-HF. In the present 1:2 complex this motion is split into in- and o~t-of-(HF)~-plane components. The small red D F shift is appropriate for this primarily fluorine motion, and the red d9 shift reflects less repulsion for the nine D atoms with smaller amplitudes of vibration. The more weakly bound Hb-F submolecule exhibits lower uB modes than the more strongly bound Ha-F submolecule, as expected. Another interesting aspect of the spectrum of the 1:2 complex is the sharp ula band at 1115 cm-' and D F counterpart at 753 cm-' with a 1.481 isotopic ratio. These are the strongest librational modes in the spectra, and annealing associates each one with the 1:2 complex. At first the high isotopic ratio is surprising, but the u3 mode for HC12- exhibits an even higher 1.502 ratiozs and v3 for HF; also has a relatively high 1.420 ratio.z5 This means that there is substantial positive quartic character in the via potential function, which probably arises from a sharpening of the potential function by repulsions from the nine hydrogen atoms on the methyl groups. Using the quartic contribution to the energy from first-order perturbation theory to fit the two isotopic ula values, as outlined previou~ly,~~ gives harmonic and quartic constants for this H-F libration and characterizes the motion as 73% harmonic and 27% quartic from the energy distribution. The q b bands are broader, probably reflecting in- and out-of-plane contributions, and the isotopic ratio is more typical (1.28 for the d9 species). Finally, it should be noted that the via mode for TMA- -(HF), at 1115 cm-I is lower than the 1227-cm-' bending mode25for HF;. Base submolecule modes for the 1:2 complex probably contribute to the V g c and vzoc bands associated with the 1:l complex. The vla(HF)mode at 1115 cm-' mixes slightly with vi: (CH, rock) in the h9 complex, resulting in a small red-shift (to 1020 cm-I) that is reversed with DF (vl$ = 1026 cm-I) owing to the opposite (and weaker) interaction with ula(DF) at 753 cm-'. In addition, the large blue-shift in the u~~mode (sym N-C, deformation) for the 1:2 complex probably arises from repulsions between the methyl groups and the Hb-F submolecule. It should be noted that the vua interval is near the u7c base submolecule mode and some mixing of these symmetric motions may occur. Larger Substituted Amines. The ethyl amine-hydrogen fluoride complex series follows the above methylamine studies, but less (28) Noble, P. N.; Pimentel, G.C. J . Chem. Phys. 1968,49, 3165. The case for reassignment to HC12- has been reviewed: Andrews, L. Annu. Rev. Phys. Chem. 1979, 30, 7 9 . (29) Smith, D. W.; Andrews, L. Spectrochim. Acta Part A 1972,28A,493.
Spectra of Amine-Hydrogen Fluoride Complexes
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4281
extensive product spectra were observed, as given in Table VII. In the triethylamine case, v,, dominated v, by an order of m a g nitude. The v, and v,, modes for diethylamine and piperidine complexes were nearly the same, as expected, showing little steric effect in these complexes. Bonding Trends. The two bonding trends of interest in this study are the effects of using different amine bases on the H F submolecule in species 1 and the nature of the (HF), subunit in species 2. The ammonia, methylamine- -HF complex series exhibited a steady and pronounced decrease in v, and increase in v, modes indicating a stronger interaction with increasing methyl substitution, which at least broadens the H-F stretching potential function and sharpens the H-F librational potential function. It can reasonably be expected that the complexes should be more strongly bound with increasing proton affinity of the amine base.30 The shift in vs from isolated HF (3919 cm-l) correlates reasonably well with base proton affinity, as can be seen from a linear plot in the thesis of Johnson.31 However, recent SCF/4-31G calculated hydrogen bond energies for the ammonia, methylamine- -HF complex series show no significant effect for methyl s~bstitution.~ It can reasonably be expected that dipole4ipole attraction makes a major contribution to the hydrogen bond energy and to the perturbation of the H-F stretching fundamental, and in this regard the dipole moment decreases steadily from ammonia (1.47 D), MMA (1.31 D), DMA (1.03 D) to T M A (0.61 D).32 This decrease in dipole-dipole attraction may counterbalance the effect expected from increased proton affinity and result in a smaller methyl effect on the dissociation energy of the complexes than expected from the displacement in v, modes alone. In conclusion, methyl substitution may alter the amine- - H F interaction in such a way to increase the breadth of the H-F intramolecular potential function without substantially changing the depth of the intermolecular potential of the complex. The 1:2 complexes are all characterized by a strong band near 1900 cm-I, which is in the region of "shared" proton vibrations. The earlier observation of H3N- -(HF), at 1920 cm-' was interpretedl to indicate proton Ha shared between H3N and HF,-; the present more extensive results for TMA- -(HF), identify two strong hydrogen bonds in the 1:2 complex and more correctly characterize the complex as represented in 2. The vibrational spectra suggest that Ha is shared between N and the inside F and that Hb-F is lengthened, but Hb is still much closer to the terminal than the inside fluorine, and the F- -Hb-F subunit cannot be considered an asymmetric bifluoride ion species. The N- -Ha--F bond is better described as three center covalent rather than proton transfer with ion formation. The lack of splitting on the vla mode suggests a colinear N - -Ha- -F unit but slight bending cannot be ruled out. Structural measurements on the pyridine- -(HF), complex in the solid state" reveal the following distances: N--Ha = 0.94 A, Ha--F = 1.58 A, central F- -Hb= 1.34 A, Hb-F terminal = 0.98 A. The vibrational spectrum of TMA- -(HF), in solid argon indicates a lesser degree of Ha proton transfer than for the pyridine- -(HF), complex in the solid state; this difference may be due more to the solid state as TMA has the higher proton affinity by 4 kcal/m01.~~ Ab initio calculations are consistent with the bonding model deduced from the vibrational spectrum for species 2 and suggest that weak interaction between the terminal subunits leads to an acute N-F,-Fb angle.33 A comparison should be made between species 1 and 2 and the analogous acetone complexes,34since acetone is only slightly less basic than ammonia in the gas phase.30 In the case of the acetone--HF complex, v, = 3302 cm-' and vl = 778 and 756 cm-' in solid argon characterizing a slightly weaker complex than TMA--HF. In the case of acetone--(HF), v,, = 2785 cm-', v,b (30) Aue, D. H.; Webb, H. M.; Bowers, M. T. J . Am. Chem. SOC.1975, 97, 4137; 1976, 98, 311. ( 3 1) Johnson, G . L., Ph.D. Thesis, University of Virginia, Charlottesville, VA. 1983. (32) 'Selected Values of Electric Dipole Moments", Nat. Std. Re5 Dura Ser. (US.Nut. Bur. Std) 1967, 10 (3). ( 3 3 ) Kurnig, I. J; Szczesniak, M. M.; Scheiner, S., to be published. (34) Andrews, L.: Johnson, G.L. J . Phys. Chem. 1984.88, 5887.
= 3565 cm-I, vla = 974 and 833 cm-', and Vlb = 657 and 613 cm-', which characterize a stronger complex than acetone- - H F but a weaker complex than TMA- -(HF)2. Clearly the base plays a major role in the strength of both 1:l and 1:2 base-acid complexes with hydrogen fluoride. The nitrogen matrix v, band positions relative to the argon matrix values provide information on the strength of the matrix interaction with the complex and on the polar character of the complex itself. The ammonia- -HF complex v, mode red-shifts 263 cm-' from argon to nitrogen,' and this red-shift decreases to 183,97, and 97 cm-' for substitution of 1, 2, and 3 methyl groups, respectively, indicating a weaker matrix interaction and suggesting substantially less polar character for TMA- -HF than H3N--HF. Part of this follows the lower dipole moment of TMA, but part may arise from a smaller dipole moment enhancement in the TMA- -HF complex as well. The 1:2 complex v, modes blue-shift from argon to nitrogen,35 and this blue shift is smallest with the TMA- -(HF), complex; the v,b mode for the latter complex is also blue-shifted by the nitrogen environment. This suggests that interaction of Hb-F with the nitrogen matrix reduces the effect of the Hb-F submolecule on the Ha-F submolecule in the complex. For this reason the amine- -(HF), interaction as a whole is not enhanced by the nitrogen matrix, in contrast to the amine- - H F interaction. Notice that the hydrogen bond stretching vu intervals for TMA--(HF)2 were unchanged within f 6 cm-I in argon and nitrogen matrices. Although there is an irregular trend for v,, in solid argon as a function of methyl s u b ~ t i t u t i o n v,,~ ~decreases steadily from 1978, 1945, 1922, to 1910 cm-' in solid nitrogen, demonstrating the same trend as in the 1:l complexes in both matrices. The methyl-substituent effect on v,, in the 1:2 complex is apparently moderated by the adjacent H b F submolecule.
Conclusions Infrared spectra of methyl-substituted amine-hydrogen fluoride complexes have been recorded in solid argon and nitrogen matrices. Concentration and sample annealing studies showed that distinct 1:l and 1:2 complexes were formed for each amine base. The 1:l complexes in solid argon revealed decreasing v, (HF stretching) and increasing vI ( H F librational) modes with increasing methyl substitution, and this trend was enhanced in solid nitrogen. These observations indicate a stronger base-acid interaction with increased methyl substitution, but this interaction may have more effect on the shape of the H-F intramolecular potential function than the base-HF intermolecular energy. Strong substantially blue-shifted -NH2 and -NH wagging complex bands were observed for the M M A and DMA complexes, and smaller perturbations on N-C stretching modes were found. These base submolecule modes exhibited 1-16-cm-' HF to DF shifts as interaction with the acid submolecule librational modes changed appreciably upon D F substitution. The relative yield of 1:2 complexes increased markedly with methyl substitution. In TMA- -(HF), both HF stretching fundamentals v,, and Vsb were observed along with extensive combination progressions in the hydrogen bond stretching modes urn and vub, following the example set by HFF. These characteristic modes are intermediate between values for CH,CN- -HF and H F y and define very strong hydrogen bonds in the TMA- -(HF), complex, which are, however, not strong enough to support proton transfer to give a trimethylammonium bifluoride salt. The vibrational spectra show that Ha is shared between N and the inside F probably closer to N and that Hb-F is elongated with Hb still closer to the terminal fluorine. The present 1:2 complexes bridge the gap between gas-phase 1:l complexes and higher order hydrogen fluoride salt complexes. Finally, the observation of the V , + vu sum band for TMA- -HF and extensive sum bands involving us,, v,b, and vu, for TMA-(HF), demonstrates anharmonic coupling between v, and vu (35) New experiments with H F and NH3 in solid nitrogen reveal 1978(more intense) and 1876-cm-I bands for the uSB mode in the NH,--(HF)* complex. (36) This may be caused by matrix cage effects on these rather larger complexes.
J . Phys. Chem. 1986, 90, 4282-4286
4282
motions and provides support for the "frequency modulation" theory of v(HX) bandwidth in hydrogen bonding.37 The vs, v,,, and vib fundamentals are relatively sharp (fwhm = 10 cm'') at 12 K in solid argon, and the sum bands are resolved. Such relatively small bandwidths for hydrogen-bonded H-F stretching fundamentals may be a consequence of well-defined structure and (37) Sheppard, N. Hydrogen Bonding. Hadzi, D., Ed.; Pergamon: London, 1959.
low-temperature quenching of rotational and vibrational motions for the complexes. Acknowledgment. We gratefully acknowledge financial support for this research from the National Science Foundation and most helpful communications and discussions with c. E. Dykstra, K. M. Harmon, D. Mootz, and S. Scheiner. Registry No. MMA, 74-89-5; DMA, 124-40-3; TMA, 75-50-3; P, 110-86-1; MEA, 75-04-7; DEA, 109-89-7; TEA, 121-44-8; Ar, 744037-1; N2, 7727-37-9; HF, 7664-39-3; D2, 7782-39-0.
Linear Polarized Birefringence Photosetection Method and the Nature of Hydrogen Bonding in Dibucaine C . T . Lin Department of Chemistry, Northern Illinois University, DeKalb, Illinois 601 15 (Received: March 13. 1986)
A birefringence photoselection method is described. The method is demonstrated for the linear polarized emission spectra of dibucaines in various solvents at 77 K. The linear polarized birefringence signals are induced and observed to superimpose on the broad emission spectra of neutral, hydrogen-bonded, monocation and dication dibucaines. Using a proper thickness of retardation plate (e.g., sapphire) gave observed birefringence signals that were useful for identifying the spectral shifts observed in the hydrogen-bonded or protonated dibucaine complexes. These shifts are interpreted as resulting from the differences in bonding strengths between the ground and excited states or the redistributions of the vibronic band intensities.
I. Introduction The influence of hydrogen bond formation on the electronic transitions of carbonyls and N heterocyclics in fluid solutions has been examined extensively.'-8 It is shown'13 that the spectral blue shift for a n,r* state and red shift for a r,r*state are generally observed for electronic absorption and emission bands on changing from a hydrocarbon to a hydroxylic solvent. Kaska' demonstrated that the observed spectral shift in the hydrogen-bonded complex is a measure of relative hydrogen bond strength between the ground and excited electronic states. On the other hand, Pimente12 argued that the apparent spectral shift found in hydrogen-bonded solution might well arise from the changes in Franck-Condon factors upon complexation. In almost all studies so far the shifts in frequency or changes in position of the n,r* or r,r*bands were measured by the difference between the point of maximum intensity of a vibrational band envelope in a hydrocarbon solvent and the point of maximum intensity of a generally featureless band in a hydroxylic solvent. This makes it impossible to measure the shift in the 0,O band upon the formation of hydrogen bonding. Moreover, the role of the Franck-Condon principle (Le., the redistribution of spectral intensity between vibronic subbands) is also difficult to infer. Recently, we have investigateds the effects of solvent on the photophysical properties of dibucaines a t 77 K. Dibucaine (2butoxy-N- [2-(diethylamino)ethyl]-4-quinolinecarboxamide) has structure I. Four different dibucaines were prepared: (1) neutral dibucaine (free base as in I), dibucaine in methylcyclohexane; (2) hydrogen-bonded dibucaine, dibucaine in ethanol; (3) monocation dibucaine (the tertiary amine N is protonated), dibucaine.HC1
__ (1) Kasha, M. Discuss. Furuduy SOC.1950, 9, 14. (2) Pimentel, G. C. J . Am. Chem. SOC.1957, 79, 3323. (3) El-Sayed, M. A,; Kasha, M. Spectrochim. Acta 1959, 15, 758. (4) Nicol, M. F.Appl. Spectrosc. Rev. 1974, 8, 183. (5) Lin, C. T.; Stikeleather, J. A. Chem. Phys. Lett. 1976, 38, 561. (6) Beecham, A. F.; Hurley, A. C. Aust. J . Chem. 1979, 32, 1643. (7) Carrabba, M. M.; Kenny, J. E.; Moomaw, W. R.; Cordes, J.; Denton, M. J . Phys. Chem. 1985, 89, 674. (8) Lin, C. T.; Malak, M.; Vanderkooi, G.; Mason. W. R. J . Phys. Chem.. submitted for publication.
0022-3654/86/2090-4282$01 . S O / O
I
in water; and (4) dication dibucaine (the tertiary amine N and the aromatic N are protonated), dibucaine.HC1 in 0.5 N H2S04 solution. All these dibucaines display a broad and structureless fluorescence emission but a well-resolved phosphorescence spectrum at 77 K. Upon hydrogen bonding and protonation, the shift in the 0,O band and the vibronic intensity redistribution in the Franck-Condon envelope of the phosphorescence spectra of dibucaines is well characterized. It was impossible to disclose the nature of the fluorescence emissions of dibucaines because of their lack of structure. In this article, a linear polarized birefringence photoselection technique is described. In this technique, a retardation plate is introduced and used to generate well-defined birefringence signals on the fluorescence and phosphorescence spectra of dibucaines. Several important characteristics of the induced birefringence signals will be discussed: (i) the observed birefringence signals originate from the nature of the retardation plate employed; (ii) when a specific type of plate with a proper thickness is used, the plate can serve as a half-wave plate and the observed spacing (Aw in cm-I) in the birefringence signals can be controlled to satisfy the condition Aw (cm-I) = > / k
(1)
where k = 1, 2, 3, ... and t (in cm-') is the frequency of the active vibrational mode of the molecule studied; and (iii) when birefringence signals with a well-defined Aw are used to deconvolute the emission spectra of dibucaines at 77 K, it is found that the linear polarized birefringence signals can reproduce the wellcharacterized features of the phosphorescence emission of dibucaines. More importantly, the birefringence signal can also be 0 1986 American Chemical Society
