J. Phys. Chem. 1984, 88, 5887-5893 charged imidazole ring and a neutron ring, i.e., soliton motion, is thus expected to occur also rapidly along the hydrogen-bonded chains in the crystal. Upon application of an electric field, the positive and negative solitons in a properly oriented chain will be able to move down the chains toward the electrodes and carry a current (Figure 7). However, the movement of the solitons modifies the orientation of the rings in the chain so that no other positive (negative) soliton can pass toward the negative (positive) electrode. In order for the current to keep flowing as is observed experimentally, the rings have to return to their original orientation in the chain (Figure 7). Considering the deformation of the lattice (26) Ralph, E. K.; Grunwald, E. J. Am. Chem. SOC.1968, 90, 517
5887
that must occur before an imidazole molecule can rotate about an axis in the plane of the molecule and perpendicular to the hydrogen-bonded chain in the crystal, Kawada et al.', have considered as reasonable an activation energy of about 1.7 eV corresponding to the one found in the conductivity vs. temperature experiments. The reorientation process could thus correspond to the limiting step for the protonic conductivity in imidazole crystals.
Acknowledgment. We acknowledge fruitful discussions with J. G. Fripiat and V. P. Bodart. J.L.B. is deeply indebted to the Belgian National Fund for Scientific Research (FNRS) for continuous support. H.C. acknowledges support from the Polish Academy of Sciences. Registry No. Imidazole, 288-32-4.
Fourier Transform Infrared Spectra of Simple Carbonyl-HF Complexes in Solid Argon Lester Andrews* and Gary L. Johnson Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: May 21, 1984; In Final Form: July 24, 1984) Co-condensation of acetone, acetaldehyde, and formaldehyde with H F and DF in excess argon at 12 K produced 1:l hydrogen-bondedcomplexes for FTIR spectroscopicstudy. The matrix absorption bands were sharp; base submoleculemodes were resolved from the base spectrum, two nondegenerate librational modes were observed, shifts in acid submolecule modes were observed for (CD,),CO substitution, and shifts in base submolecule modes were found for DF complexes. Blue shifts for the in-plane C-C-O deformation modes in the acetaldehyde and acetone complexesverify that the HF ligand is attached to oxygen in the skeletal plane of the base submolecule. Two different H-F fundamentals were observed for the 1:2 acetone- -(HF), complex which indicates a chain arrangement of the acid ligands.
Introduction Carbonyl compounds are important as synthetic precursors and solvents, and their properties depend a great upon the electron-rich oxygen atom in the carbonyl group. Formaldehyde, the simplest carbonyl compound, is a logical starting point for a systematic study of hydrogen bonding to the carbonyl group. The microwave spectrum' of the H,CO--HF complex revealed a coplanar structure with HF attached to oxygen and a C=O- - - F angle of 1 loo. Matrix infrared spectra of this complex provide complementary information and show a substantial perturbation on the carbonyl stretching vibration.,^^ In addition, gas-phase spectra of 1:l and 1:2 complexes of acetaldehyde and acetone with H F have been r e p ~ r t e d . ~It, ~is of particular interest to determine the effect of a second HF ligand on the 1:l complex, and the matrix isolation technique is well-suited for the preparation and characterization of 1:l and 1:2 complexes of HF and carbonyl base molecules. Since matrix absorptions are sharp, useful information can be obtained from base submolecule modes that are resolved from the base spectrum and two different librational modes for complexes with less than threefold symmetry, as is the case with the present carbonyl complexes.
spectrum for each experiment. Wavenumber accuracy is better than *1 or *0.2 cm-I for bands reported to the nearest 1 or 0.1 cm-I, respectively. The H F and D F samples were prepared by reacting at room temperature about 0.5 mmol each of F, (Matheson) and H, (Matheson, research grade) or D, (Air Products) in a 3-L stainless steel can and diluting the product with argon (Burdett, 99.995%) to concentrations between 300/ 1 and 600/ 1 Ar/HF mole ratio. The acetaldehyde (Baker), acetone (Mallinkrodt), and acetone-d6 (Merck) were outgassed by repeated condensation and thawing under high vacuum, and samples were prepared by diluting about 0.25 mmol of reagent with argon to 300/1 and 600/1 Ar/reagent mole ratios. The argon solutions of H F and acetaldehyde (or acetone) were deposited onto a CsI window at 3-5 mmol/h each for up to 20 h to form the doped Ar matrix. Formaldehyde was deposited neat through a needle valve from a 5-mm i.d. stainless steel tube packed with paraformaldehyde (Eastman) and heated to 63-68 OC, while a stream of Ar (doped with HF or DF) was simultaneously sprayed onto the cold window. Samples were annealed by warming to 22-24 K for 10 min and recooling to 12 K and additional spectra were recorded.
Experimental Section The closed-cycle cooler and stainless steel vacuum system have been described Spectra were recorded on a Nicolet 7199 FTIR spectrometer at I-cm-' resolution between 4000 and 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 the matrix to produce a simulated double-beam
Results Experiments performed with hydrogen fluoride and formaldehyde, acetaldehyde, and acetone will be described. Formaldehyde. A formaldehyde-doped argon matrix was prepared and found to be similar to that reported previously.* Annealing the sample produced a 1737-cm-I shoulder on the 1742-cm-' C=O fundamental without changing the other H2C0 bands. Similar experiments were done with HF in the matrix, and the new product bands are illustrated in Figure la. A strong sharp band was observed at 3570 cm-' (labeled us) with 3583- and 3555-cm-' satellites (the latter labeled Wc is due to H20--HF)?
(1) Baiocchi, F. A.; Klemperer, W. J . Chem. Phys. 1983, 78, 3509. (2) Johnson, G. L. Ph.D. Thesis, University of Virginia, 1983. (3) Bach, S. B. H.; Auk, B. S. J . Phys. Chem. 1984, 88, 3600. (4) Arnold, J.; Millen, D. J. J . Chem. SOC.1965, 510. (5) Couzi, M.; LeCalve, J.; Van Huong, P.; Lascombe, J. J . Mol. Strucr. 1970, 5, 363. (6) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76, 2875. (7) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J . Chem. Phys. 1982, 76, 5767.
0022-3654/84/2088-5887$01.50/0
(8) Khoshkhoo, H.; Nixon, E. R. Specrrochim. Acta, Parr A 1973, 2 9 4 603; Nelander, B. J . Chem. Phys. 1980, 72, 77. (9) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1983, 79, 3670.
0 1984 American Chemical Society
5888 The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
1
Andrews and Johnson
Y
WAVENUMBERS
Figure 1. (a) Infrared spectrum of a matrix formed by deposition of 92 mmol of Ar/HF = 500/1 and H2C0vapor; (b) infrared spectrum of a matrix formed by codeposition of 83 mmol of Ar/DF = 400/1 and H2CO vapor; the DF contained approximately 20% HF.
a
26ba
25ba
2nbo
I
WAVENUMBERS
Figure 2. (a) Infrared spectrum of 50 mmol of Ar/HF = 300/1 deposited with 33 mol of Ar/CH,CHO = 300/1; (b) infrared spectrum of 35 mmol of Ar/DF = 300/1 codeposited with 43 mmol of Ar/CH,CHO = 300/1; the DF contained approximately 15% HF.
and a sharp doublet was observed at 612, 602 cm-I (labeled uI). In the region of HzCO absorptions, a strong, very sharp 1731.4-cm-l band (fwhm = 1 cm-', labeled uZc) was completely resolved from the HzCO fundamental at 1742.2 cm-', and no other H2C0 bands exhibited new satellites. An experiment with D F gave the spectrum shown in Figure 1b; the u, and u, bands shifted to 2639, 2630 cm-' and 462,454 cm-l, respectively, and the sharp vzc band shifted to 1730.9 cm-I in two experiments. Sample annealing to 22 K doubled the product absorbances and produced
weak new bands in the low-frequency region. Acetaldehyde. Six experiments were done with acetaldehyde; spectra are compared in Figure 2 for H F and D F experiments and product bands are listed in Table I. The major us band was observed at 3416 cm-I with a resolved satellite at 3369 cm-I, and uI bands appeared at 722 and 693 cm-I with an intermediate splitting at 712 cm-l and weaker satellites at 781 and 766 cm-'. In addition to the three uc bands shown in Figure 2, another was observed at 1353 cm-l above the precursorlo band at 1349 cm-l.
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5889
FTIR Spectra of Simple Carbonyl-HF Complexes
3~kT?&CXaa
i e b o a a isba
irba
i3ba
'izbo
itbo 'iobo
gbo
sbo
7ba
sba
sbo
+bo
WAVENUMBERS
Figure 3. (a) Infrared spectrum of a matrix formed by codepositing 54 mmol of Ar/HF = 300/1 with 40 mmol of Ar/(CH,)2C0 = 300/1 at 12 K; (b) infrared spectrum of a matrix formed by codepositing 46 mmol of Ar/HF = 300/1 with 38 mmol of Ar/(CD3)2C0 = 300/1 at 12 K. TABLE I: Absorptions (cm-I) Produced upon Co-condensation of Acetaldehyde and HF and DF in Solid Argon at 12 K HF DF assignment 3416 3369 1722 1353 1129 78 1 766 722 712 693 532.0
2527 2492 1722 1353 1129 58 1
3517
2595 2236
558 55 1 518 529.3
US
us (site)
uC (C=O str) (1728)' uc (CH3 def) (1349)
uE (C-C str) (1111) u1 (site)
uI (site) uI (in-plane) uI (in-plane)b u1 (out-of-plane) uc
(C-C-0 def) (506)
us (Hb-F) us
(Ha-F)
'Argon matrix absorptions of acetaldehyde. bProbably due to slightly different matrix packing in molecular plane than the 722-cm-I
band. TABLE 11: Absorptions (cm-I) Produced upon Co-condensation of Acetone with HF and DF in Solid Argon at 12 K HF DF assignment 3333 2472 us (site) 3302 2452 US 1715 1713 u2(al) C=O str (1722)' 1423 1423 uqC(al)CH, def (1429) 1374 1374 u16C(bl)CH, def (1361) 1242 1242 u12c(bl)C-C str (1216) 1097 1097 u2zc(b2)CH, rock (1092) 778 597 770 756 736 556 3565 2785 974 833 657 613
'Argon matrix absorptions of (CH3)2C0. bobscured by the 557-
cm-I
u,
band.
Sample annealing in H F experiments decreased HF, doubled the above product bands and the (HF), band at 3702 cm-', and
TABLE 111: Absorptions (cm-I) Produced upon Co-condensationof Acetone-d6 with HF and DF in Solid Argon at 12 K &+HF d,+DF assignment 3331 2470 us (site) 3299 2450 US 1266 1266 ulSC(bl)C-C str (1240)O 969 969 u22C(b2)CD, rock (963) 895 895 ugC(al)CD, rock (885) 780 584 Y1 770 574 uI (site) 761 555 VI 742 543 u1 (site) 506 496 u19C(bl)C-C-0 bend (475) 3565 2785 967 829 657 613
263 1 2112 706 635 b 448
us(Hb-F) u,(H,-F) VI (Ha-F) VI (Ha-F) (Hb-F) uI(Hb-F)
'Argon matrix absorption of (CD,),CO. cm-I precursor band.
bobscured by the 475-
produced a sharp new 3517-cm-l band and broad 3244, 3186, and 3118 cm-' bands due to cyclic (HF), species." In DF-doped samples, the major us band was observed at 2527 cm-I with a resolved satellite at 2492 cm-I, and vI bands appeared at 558 and 518 cm-I with a weaker satellite at 581 cm-I. The higher-frequency uC bands were unchanged with DF, but the lower-frequency product band at 529.3 cm-' exhibited a small DF shift. Sample warming decreased DF, doubled the above product bands and (DF), at 2717 cm-', and produced a sharp new 2595-cm-' band, a broad new 2236-cm-' band, and bands due to cyclic (DF), species." Acetone. An extensive series of experiments was done with acetone-h, and -d6 and essential features of these samples codeposited with H F are shown in Figure 3. The strong us band at 3302 cm-I with a 3333-cm-I satellite shifted 2 em-' with d6 substitution in the precursor and the u, bands at 778 and 756 cm-' with satellites at 770 and 736 cm-' exhibited small shifts that can be seen from the data in Tables I1 and 111; the product bands labeled uc in Figure 3 are associated with precursor fundamentals,I2 (10) Hollenstein, H.; Gunthard, Hs.H. Spectrochim. Acta, Purr A 1971, 27A, 2027. (1 1) Andrews, L.; Johnson, G. L. Chem. Phys. Lett. 1983,96, 133; J . Phys. Chem. 1984,88,425. (12) Dellepiane, G.; Overend, J. Spectrochim. Acta 1966, 22, 593.
5890
3dflO
Andrews and Johnson
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
3Cb0
32bfl 3000 WAVENUMBERS
280fl
10-
Figure 4. Infrared spectra of a matrix containing acetone and hydrogen fluoride. (a) Ar/HF = 300/1 and Ar/(CH3)*C0 = 300/1 samples codeposited at 12 K (b) after warming to 20 K for 10 min and recooling to 12 K. A denotes acetone.
as listed in the tables. These new product absorptions were not present in H F or acetone samples deposited separately. Spectra for acetone deposited with HF and DF are contrasted in Figures 4a and 5a and Table 11. The strong v, band for DF at 2452 cm-I had a sharp satellite at 2472 cm-', and the two V I bands revealed similar satellites. Figures 4b and 5b illustrate the effect of sample annealing. The vs, v,, and vc bands discussed above doubled in absorbance and acquired additional shoulders while weaker bands at 3565,2785,974,833,657, and 613 cm-' in Figure 4b increased fivefold as did the DF counterparts in Figure 5b. An experiment was also done with acetone-d6 and DF and the product bands are listed in Table 111. Particular attention is called to the d6 shifts on the D F v1 modes and the HF-DF shift in the vIgc(bl)acetone-d6 submolecule mode.
Discussion The new product absorptions observed here will be assigned to 1:1 and 1:2 complexes between carbonyl bases an8 HF. Base submolecule modes in the complexes will be employed to describe the bonding site of the HF ligand. Acetone. Two sets of product bands were observed in these experiments when HF and acetone were codeposited: the first set, strong in the initial sample deposit, doubled on sample annealing to allow diffusion and further association of HF, and the second set, weak in the deposited sample, increased fivefold on sample annealing. These observations clearly demonstrate that two different complexes were formed between acetone and HF. The strongest band in the first set at 3302 cm-l exhibited a DF counterpart at 2452 cm-'; these bands showed small (2-3 cm-I) shifts when acetone-d6 was employed, which confirms the formation of an acetone-HF complex. The 3302-cm-' argon matrix band corresponds to the 3460-cm-' maximum in gas-phase ~ p e c t r a ,which ~ , ~ has been assigned to the vs (H-F) fundamental in the 1:l complex (CH3)&O- -HF (1). Attractive interaction H3C
\.c=o. I
between the solid argon matrix and the complex is responsible
I 27b0
23b0 2100 WOVENUMBERS
25bO
EbO
600
$bo
Figure 5. Infrared spectra of matrix containing acetone and 90% enriched deuterim fluoride: (a) Ar/DF = 150/1 and Ar/(CH&CO = 300/1 samples codeposited at 12 K, (b) after warming to 24 K for 10 min and recooling to 12 K.
for a substantial reduction in half-width (to 30 cm-') and a 158-cm-I matrix shift. This red matrix shift is intermediate between 53- and 174-cm-' values found for the weaker H 2 0and ~ ~ ~v,(HF)/v,(DF) ~ stronger NH3 c o m p l e x e ~ . The fundamental ratios for the gas phase (1.352) and matrix (1.347) suggest more anharmonicity in the latter, which could be due to attractive matrix interactions leading to a slight H-F bond elongation. The 778- and 756-cm-I bands in solid argon are assigned to the in- and out-of-plane librational modes v1 of H-F in the acetone-HF complex where the acetone skeleton defines the plane of reference; these modes gave a single broad 695 cm-' band in the gas phase.5 This blue matrix shift is due to the repulsive effect of the matrix cage on the librational motion. The different v,(HF)/uI(DF) ratios 778/597 = 1.303 and 756/557 = 1.357 are consistent with two different H-F motions, although the former is unusually small for a typical vI mode (HF/DF ratios for the v, modes in the (CH3)*0--HF complex are 1.366 and 1.363),14 which suggests interaction with another normal mode. Six different base submolecule modes were observed for the complex. Three of these involve small methyl group mode shifts both above and below the acetone absorptions,12and three involve l a f g q shifts of skeletal modes which provide information about the H-F position in the complex. The 7- and 26-cm-' red displacements of v3(al) and v12(bl),the @2=0and antisymmetric C-C stretching modes, point to the carbonyl oxygen as the bonding site. The 2-cm-' shift of v(C=O) to lower energy in the (CH3),CO- -DF complex is probably due to interaction between v,(DF) at 2452 cm-' and v(C=O) at 1713 cm-' and also supports HF attachment to oxygen in the complex. The weak 3400-cm-' product band below the 2v3(C=O)%and of acetone at 3430 cm-' could be due to the overtone 2v3c of the complex, although growth in this region on annealing is probably due to cyclic (HF),. If the weak 3400-cm-' band is due to 2vJC,there is no evidence of intensity enhancement from Fermi resonance with Y, at 3302 cm-', which casts doubt on 2v3cas a contribution to broadening of v, in the gas phase, as discussed by Millen in a recent review.15 (13) Johnson, G.L.; Andrews, L. J . Am. Chem. SOC1982, 104, 3043. (14) Andrews, L.; Johnson, G . L.; Davis, S. R., to be published.
FTIR Spectra of Simple Carbonyl-HF Complexes
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5891
These observations rule out a ?r carbonyl complex which would be expected to absorb in the 3700-cm-I region, as found for C2H4--HF in solid argon,7 and cast doubt on the contribution of a ?r complex in the gas phase as a source of band b r ~ a d e n i n g . ~ Furthermore, the 27-cm-' blue displacement of the C-C-0 skeletal bending mode v19 (b, in-plane)l2confirms attachment of H-F to oxygen and indicates that the H-F ligand is in the skeletal plane of the acetone submolecule. Similar acetone-d6 submolecule absorptions were observed although the C=O stretching fundamental for the complex was not resolved from the acetone-d6 absorption. The C-C-0 bend for the d6 submolecule in the HF and D F complexes exhibited a 10-cm-' shift due to different interaction and mixing with the HF and DF librational modes. This interaction provides a basis for identification of the 778-cm-' band as the v l (in-plane) mode and the 756-cm-' band as the v, (out-of-plane) mode. The vlmodes for the (CD3),CO- -HF and - -DF complexes are slightly shifted from the h6 values as can be seen in Tables I1 and 111. The 780/584 = 1.336 and 761/555 = 1.371 ratios for the u1 modes in the d6 complex are also different from the HF/DF isotopic ratios for the h6 complex given above. The major difference in these ratios is due to a greater interaction between the 597-cm-l h6 vI(DF) mode and vl; at 556 cm-' than for the 584-cm-' d6 vl(DF) mode and vl$ at 496 cm-' due to the red d6 shift in v19 for acetone. This interaction results in a higher ul(DF) h6 mode and a correspondingly lower v,(HF)/v,(DF) ratio for the h6 complex. The v l 9 (in plane) C-C-0 bending mode thus interacts with the vI (in-plane) librational mode and alters this HF/DF ratio but not the H F / D F ratio for the v1 (out-of-plane) mode. Accordingly, the higher-frequency librational mode is the in-plane component and the lower mode is the out-of-plane component; this assigment is expected on the basis of electrostatic interaction between the acetone and acid hydrogens. The weaker 3333-cm-' satellite above the strong v, band and the weaker 770 and 736-cm-' satellites below the vI modes are appropriate for a different less strongly interacting matrix site for 1. Such a site could involve less intimate packing of argon atoms around the H-F ligand. This explanation is consistent with the sample warming behavior which showed more growth in the strong v, and vl absorptions due to 1 in the major site in solid argon. Finally, some comment about the considerable sharpening in the strong v, band and the absence of a lOO-cm-' progression of blue shoulders observed in the gas p h a ~ eis~ appropriate. ,~ The 12 K matrix cage removes contributions from low-frequency hot bands and provides indirect evidence that combination hot band series contribute to the gas-phase bandwidth as discussed by Millen15 and Thomas,16 although recent studies of Schriver et al.17 show that bandwidth is a complicated problem. The second group of bands contained sharp 3565-cm-l and broader 2785-cm-l features with D F counterparts at 2632 and 2114 cm-I. The HF/DF ratios, 1.354 and 1.317, bracket the 1.347 ratio for v,(H-F)/v,(D-F) for the 1:l complex and fall below the 1.365 ratio for the diatomic molecule in solid argon. This behavior is appropriate for H-F motions in different hydrogen-bonded configurations, which is expected for the 1:2 complex 2. The n3c\
/=a, H3C
" P
\F- -H
b-
F
2
3565- and 2785-cm-I matrix bands are in agreement with sharper 3665-cm-' and broader 3050-cm-I bands observed in H F rich gas-phase mixtures and assigned to 2.5 The 100- and 265-cm-l red matrix shifts for the H-F fundamentals in 2 bracket the 158-cm-' value for 1. The 2785-cm-' band is due to the v(Ha-F)
stretching mode in 2 reduced from the v, value in 1 owing to the fluoride ion affinity of Hb-F. The 3565-cm-l band for the v(Hb-F) stretching mode is substantially lower than the u(Hb-F) mode" in (HF), at 3826 cm-' owing to the cooperative hydrogen-bonding effect of the strong acetone base attached to proton Ha. Two librational modes were observed for each H-F submolecule in 2; the higher-frequency pair is due to the more strongly bound Ha-F submolecule and the lower frequency pair is assigned to the more weakly bound Hb-F submolecule. Base submolecule modes for 2 were observed as additional shoulders on these modes observed for 1 giving a larger displacement from the acetone values. Acetaldehyde. The matrix spectra of acetaldehyde and H F condensation products were similar to the above acetone observations except 1:2 complex bands were much weaker. The major absorption at 3416 cm-l with a DF counterpart at 2527 cm-l (v,(HF)/v,(DF) = 3416/2527 = 1.352) is due to the v,(H-F) stretching mode in the CH3CHO--HF complex 3. This matrix /F
HSC, H 2=O,\
H3C\ H /c=o
H
3b
\F
3a
band is red-shifted from the broad 3550-cm-' gas-phase fundamentaL5 The 722- and 693-cm-I bands are assigned to vI(H-F) modes in 3; these modes gave a single broad 645-cm-' gas-phase band.5 Again, the different vI(HF)/vl(DF) ratios 722/558 = 1.294 and 693/518 = 1.338 indicate two different motions and interaction of one y mode with the C-C-0 deformation model0 of the complex. Four acetaldehyde submolecule modes were observed for the complex as listed in Table I. The C 4 and C-C stretching modes were shifted 6 and 16 cm-l, respectively, lower in the complex whereas a methyl deformation mode was shifted 4 cm-I higher. The C-C-0 deformation mode was 26 cm-' higher in the H F complex, but only 23 cm-' higher in the D F complex. This DF isotopic shift for the in-plane C-C-0 deformation mode arises because of interaction with the in-plane uI(D-F) mode, which forces the vl(D-F) mode slightly higher and the C-C-0 deformation mode slightly lower. Accordingly, the lower HF/DF ratio identifies the in-plane uI mode, which is in part due to this interaction between acid and base submolecule in-plane vibrational modes. A very strong resolved satellite was observed at 3369 cm-' in the ys region; a similar D F band at 2492 cm-' gave an identical HF/DF ratio to the major us band. The larger displacement for the 3369-cm-' satellite is indicative of a stronger hydrogen bond or matrix interaction than the major 3416-cm-' band. A weaker pair of vl bands at 781 and 766 cm-l is also appropriate for the species giving the 3369-cm-' v, band. Two possible explanations for t h e & t y o g t s of v, and v1 bands are (1) two matrix sites one with a strongeimatrix interaction or (2) different structures like 3a and 3b with the H F ligand opposite or adjacent to the methyl group. Sample warming doubled the above 1:l complex bands and produced evidence for a 1:2 complex. A sharp band at 2596 cm-I and a broad band at 2236 cm-' in DF experiments are appropriate for u,(Hb-F) and u,(Ha-F) modes, respectively. The H-F counterpart of the former was observed at 3517 cm-', but that of the latter was obscured by C-H modes of the precursor. Formaldehyde. The strong 3570-cm-' product in formaldehyde-HF experiments is clearly due to the u, mode of complex 4, in general agreement with Bach and A ~ l t The . ~ DF counterpart H
\
/c=o \\
H
D.J. J . Mol. Struct. 1983, 100, 35 I . (16) Thomas, R. K. Proc. R.SOC.London,Ser. A 1971, 235, 133. (17) Schriver, L.; Loutellier, A,; Burneau, A.; Perchard, J. P. J . Mol. Struct. 1982, 95, 37.
/YH
H \F
(1 5 ) Millen,
4
at 2630 cm-' gave a v,(HF)/v,(DF) ratio of 1.357, similar to the
5892
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
Andrews and Johnson
TABLE IV: Acid Submolecule Modes and Carbonyl Stretching Modes (cm-I) in 1:l Carbonyl-Hydrogen Fluoride Complexes base v,(gas)" v,(Ar) v,(gas)" v,(Ar) VC-0 (Ar) v+OC(Ar) CHzO 3570 612, 602 1742 1731 CHPCHO 3550 3416 645 722, 693 1728 1722
(CHd2CO
3460
3369' 3302 3333d
695
781, 766' 778,758 770, 736d
1722
1715
PAb 175 186 195
"Reference 5. *Reference 19, f2 kcal/mol. CComplexin more strongly interacting matrix site. dComplex in less strongly interacting matrix site. other carbonyl complexes. The two v1 modes at 612 and 602 cm-' were separated less than the vI modes in 1 and 3 and their vI(HF)/vI(DF) ratios were almost identical (1.325 and 1.326) in contrast to the former vI modes. However, in the H,CO--HF case, no lower-frequency base submolecule modes can interact with either vI motion and alter their HF/DF ratios. Following the characterization of v1 modes for 1 and 3,the higher 612-cm-' band is probably due to the vl (in-plane) and the lower 602 cm-' to the v1 (out-of-plane) librational modes. The 1 1-cm-' displacement or the C=O stretching mode in the complex, in agreement with earlier worker^,^ again suggests attachment to the carbonyl oxygen as determined from the microwave spectrum.' The OS-cm-' red shift in v ? ( C Y ) in the CH20--DF complex is due to interaction between the v,(DF) and v?(C=O) modes; this interaction is larger in the present (CH3)2CO--DFcase where v,(DF) is closer to vc(C=O) and in formaldehyde complexes with HCl and HBr.3 Structure and Bonding. The microwave spectrum of H2CO-HF indicates a coplanar complex with a C=O---F angle of 1loo. Here the H F submolecule acts as a probe for electron density around the carbonyl oxygen atom and provides evidence for the sp2 model of hybridized orbitals on oxygen, which is in general accord with a number of theoretical calculations.'* Substantial blue shifts in the in-plane C-C-0 deformation modes for the acetone submolecule in 1 and acetaldehyde submolecule in 3 show that the H-F ligand is in the base molecule skeletal plane in these complexes; this effect has been documented for the C2H4- - H F complex' where only hydrogen-bending modes in the direction of the ligand were blue shifted owing to increased electrostatic repulsion between base and ligand. Similar red shifts for the carbonyl stretching mode in complexes 1 and 3 imply H F attachment at oxygen, as found for 4. Several bonding trends are of interest here. The first is to correlate the H F submolecule modes in 1,3,and 4 as a function of base proton affinity. As can be seen in Table IV, an increase in base proton affinitylg manifests a decrease in v, and an increase in v1 which characterizes an increase in hydrogen bond strength. This does not, however, correlate with the magnitude of the displacement in the v(C=O) mode on complex formation. The in-plane and out-of-plane librational modes measure the orientational rigidity of the hydrogen bond; the stronger bond is more rigid and exhibits higher v1 modes. The observation of two nondegenerate librational modes is indicative of asymmetry in the base electron density responsible for formation of the hydrogen bond, although the v, splitting for the present carbonyl complexes is comparable to that found for ethylene (23 cm-I) and substantially smaller than that for dimethyl ether (1 14 cm-') complexes?*14 It is perhaps noteworthy that the simpler C H 2 0complex gives the smallest v1 splitting and that the asymmetric CH,CHO base gives the largest vI splitting. As can be seen from the magnitude of the displacement in us below the 3919-cm-' vibrational fundamental for H F in solid argon," the complexes observed here are relatively strong, and theoretical calculations on other strong complexes,20 such as H3N--HF, have shown the binding to be primarily electrostatic
(18) Kollman, P. A,; McKelvey, J.; Johansson, A.; Rothenberg, S.J . Am. Chem. SOC.1975, 97, 955; Gordon, M. S.; Tallman, D. E.; Monroe, C.; Steinbach, M.; Armburst. J. J . Am. Chem. SOC.1975, 97, 1326. (19) Wolf, J. F.; Staley, R.H.; Kappel, I.: Taagepara, M.; McIver, R. T., Jr.; Beauchamp, S. L.; Taft, R. W. J . Am. Chem. SOC.1977, 99, 5417.; Collyer, S. M.; McMahon, T. B. J . Phys. Chem. 1983, 87, 909. (20) Umeyama, H.; Morokuma, K. J. Am. Chem. SOC.1977, 99, 1316.
in nature with a smaller contribution from charge transfer. The matrix interaction is attractive with the polar parts of the complex, which gives rise to the 158-cm-' red shift in vs for the stronger acetone complex. A small red shift may be manifested in the v(C=O) mode for 1, which has not been observed in the gas phase, since this mode for acetone exhibited at 10-cm-' red matrix shift; however, the other skeletal base molecule modes showed small (1-2 cm-I) matrix shifts and these submolecule modes in the complex probably exhibit small matrix shifts as well. The matrix interaction is repulsive for the librational modes as the average 72-cm-' blue shift for 1 demonstrates. Based on the gas-to-matrix shifts for 1 and 3,the weaker 4 complex is estimated to have v, = 3650 f 40 cm-I and v, = 560 f 20 cm-' in the gas phase. Comparisons with the analogous (CH,)2CO- -HCl complex, which has been observed in the gas phase2I and in solid argon,22 are of interest. The latter is also a strong complex; in fact Av,/v, is 0.162 for HCl and 0.157 for H F complexes with acetone. The large gas-to-matrix shift (277 cm-', assuming the gas-phase origin is correctly identified) for v, of the HCl complex arises from a strong matrix interaction, which suggests more proton transfer in the (CH3)&O- -HCl complex than in 1. Such is the case for the HF and HCl c o m p l e x e ~ lwith ~ , ~ ~NH3, which has a slightly higher proton affinity than acetone. The larger red shift (13 cm-I) in the C=O stretching and the larger blue shift (30 cm-I) in the C-C-O bending base submolecule modes in the (CH3)2CO--HC1 complex may be due to a higher degree of proton transfer. However, the more polar H F acid should produce a stronger electrostatic interaction and more bond polarization throughout the acetone submolecule, which is manifested in a larger red shift (26 cm-') in the antisymmetric C-C stretching fundamental (16 cm-' for the HC1 complex). The 1:2 complex 2 is formed more readily in acetone experiments which gave the stronger 1:l complex. A similar trend has been found for NH, and methylamine^.'^^^ Although the addition of Hb-F to 1 substantially weakens the Ha-F bond, as manifested in the lower v(H,-F) fundamental, the effect of Hb-F on the stronger H,N- -HF complex is more marked giving a shared Ha proton.
Conclusions Co-condensation of simple carbonyl compounds and H F in argon at 12 K produced 1:1 hydrogen-bonded complexes; FTIR spectra of these complexes in solid argon are complementary with gas-phase spectra. Although an attractive matrix interaction is indicated by large red shifts in the v,(H-F) modes, hot bands are eliminated and the matrix spectra are sharp, band origins are readily identified, and base submolecule modes are resolved from the base spectrum. The vI(H-F) librational modes were blue shifed slightly by the matrix cage, and two nondegenerate bands were resolved in the matrix, which was not possible in the gas phase. The v, modes decreased and the v1 modes increased with increasing proton affinity of the base which characterizes stronger, more rigid hydrogen bonds. Carbonyl stretching modes for the CH20--HF complexes, resolved below the base absorption, exhibited small shifts when H F was replaced by DF, which demonstrates interaction between vc(C=O) and v,(D-F) modes and suggests attachment of HF to oxygen in the complexes. Larger acid isotopic (21) Bertie, J. E.; Millen, D. J. J . Chem. SOC.1965, 497. (22) Nowak, M. J.; Szczepaniak, K.; Baran, J. W. J . Mol. Srrucr. 1978, 47, 307. (23) Ault, B. S.; Pimentel, G.C. J . Phys. Chem. 1973, 77, 1649. (24) Johnson, G. L., Andrews, L., to be published.
J. Phys. Chem. 1984,88, 5893-5898 shifts were observed in the uc (C-C-0 in-plane deformation) modes for the acetaldehyde and acetone complexes on D F substitution owing to interaction with the u,(DF) librational modes; this interaction identifies the higher-frequency u, modes as in-plane and the lower v1 mode as out-of-plane. Blue shifts for the in-plane C-C-0 deformation modes in these complexes verify that the HF ligand is in the skeletal plane of the base molecule as determined from the microwave spectrum of CH20--HF.l Sample annealing increased absorptions due to a 1:2 complex with markedly higher yield in acetone experiments; two H-F fundamentals were ob-
5893
served, one above and one below the strong v, mode of the 1:l complex, which indicates one stronger and one weaker hydrogen bond in the 1:2 complex and verifies the (CH3)$0--H,-F- -Hb-F chain arrangement for the acid submolecules. Acknowledgment. Financial support from the National Science Foundation, the assistance of R. D. Hunt with several experiments, and a prepoint from B. S.Ault are gratefully acknowledged. Registry No. (CH3)*C0, 67-64-1; CH3CH0, 75-07-0; CH20, 5000-0; HF, 7664-39-3; DF, 14333-26-7.
Absorption Spectra and Photochemical Rearrangements of Alkyl- and Dialkylbenzene Cations in Solid Argon Benuel J. Kelsall and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: June 5, 1984)
Matrix photoionization of alkyl- and dialkylbenzenes produced and trapped the parent radical cations. Irradiation in the visible parent cation absorption induced a-H transfer to the cation ring to give substituted methylenecyclohexadienecations. The ease of 1,3-hydrogen transfer in these experiments suggests that this may be an important rearrangement in gaseous alkylbenzene cations. Subsequent ultraviolet photolysis of these samples produced substituted styrene cations.
Introduction Earlier papers from this laboratory have reported optical absorption spectra for a variety of substituted methylenecyclohexadiene These species were produced by photochemical rearrangement of toluene, cycloheptatriene, and norbornadiene parent cations and their methyl- or chlorine-substituted derivatives isolated in solid argon. The methylenecyclohexadiene cation is the familiar McLafferty rearrangement product, which has been produced from a number of gaseous cation precursors including the n-butylbenzene ~ a t i o n . " ~The matrix experiments differ from the gas-phase experiments in that the C7Hs+'product is produced by intramolecular rearrangement in the former and by P-cleavage of a molecular fragment in the latter. The present study extends the earlier matrix work by examining substituted methylenecyclohexadiene cations produced from alkylbenzenes with substituents larger than methyl to provide more information about the rearrangement process and the identification of the final product. The alkylbenzene cations are of interest owing to competition between rearrangement and decomposition pathways. The relative yield of photolysis products for each pathway can be used to establish relative stabilities for different cation structures and lead to a better understanding of the bonding in cation species. Experimental Section The apparatus and matrix photoionization techniques have been described in earlier papers.8-10 Two different light sources were (1) Kelsall, B. J.; Andrews, L. J . Am. Chem. SOC.1983, 105, 1413. (2) Kelsall, B. J.; Andrews, L.; McGarvey, G. J.; J. Phys. Chem. 1983,87, 1788. (3) Andrews, L.; Kelsall, B. J.; Payne, C. K.; Rodig, 0. R.; Schwarz, H. J . Phys. Chem. 1982,86, 3714. (4) Levsen, K.; McLafferty, F. W.; Jerina, D. M. J. Am. Chem. SOC.1973, 95.I ..
6332.
(5) Baldwin, M. A.; McLafferty, F. W.; Jerina, D. M. J . Am. Chem. SOC. 1975, 97, 6169. (6) McLafferty, F. W.; Bockhoff, F. M. J . Am. Chem. Soc. 1979, 101, 1783. (7) Dunbar, R. C.; Klein, R. J . Am. Chem. SOC.1977, 99, 3744. (8) Andrews, L.; Keelan, B. W. J. Am. Chem. Soc. 1980, 102, 5732. (9) Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrosc. 1978, 32, 157.
0022-3654/84/2088-5893$01.50/0
TABLE I: Band Positions (em-') and Assignments for Substituted Benzyl Radicals Isolated in Solid Argon at 20 K Formed by H-Atom Dissociation from a Given Precursor EtBe/ n-PrBel i-PrBe/ n-BuBel t-BuBel p-i-PrTI/ a-Me" a-Et a,a-diMe a-Pr a,a-diMe p-i-Prb tetralin 31660 31620 31530 31600 31540 31730 30720 32220 32200 32620 32150 33940 32290 32640 32630 32990 32 630 33160 33080 33960 33 040 34130 34050 34570 34 170 34580 34340 35010 35320 34590 35510 35520 35290 35690 35 780 36 280 36 500 36 770
"Precursor/benzyl radical product. the p,a,a-trimethylbenzyl radical.
Weaker origin at 31 450 for
used to irradiate the matrix samples: a windowless argon resonance lamp powered by a microwave discharge for vacuum ultraviolet (vacuum UV, 1 1.8 eV) irradiation during sample depo~ition,~ and a high-pressure mercury arc for ultraviolet-visible (UV-vis, 45 000-10 000 cm-l) photolysis after deposition.1° Argon/reagent or argon/reagent/CH2C12 or CCl, samples with argon in excess by 200-2000 were deposited at 2 mmol/h for 2-6 h onto a 20 f 2 K sapphire window; reagents were used as received from Aldrich. Two different irradiation schemes were employed, each with and without chlorocarbon compounds added to serve as electron traps: one format involved depositing the sample then irradiating with filtered UV-vis light, and the other procedure employed vacuum UV irradiation during sample condensation, and then filtered UV-vis photolysis of the photoionized sample. Absorption spectra were recorded on a Cary 17 spectrophotometer, samples were irradiated again with the mercury arc using Corning pyrex and colored glass filters, and more spectra were recorded. The illustrated spectra were digitized on an Apple I1 or Nicolet (10) Kelsall, B. J.; Andrews, L. J . Chem. Phys. 1982, 76, 5005.
0 1984 American Chemical Society
