Electron paramagnetic resonance and infrared spectra of


J. A. Howard, R. Sutcliffe, C. A. Hampson, and B. Mile. J. Phys. Chem. , 1986, 90 (18), pp 4268–4273. DOI: 10.1021/j100409a010. Publication Date: Au...
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J . Phys. Chem. 1986, 90, 4268-4273

4268

formation in 1,2-benzanthracene was reported to be -6.2 cm3/ mol.22 This may indicate that the IE has a looser configuration than the excimers from two free chromophores, since there will not be so large a difference between AV,* and AV,.

where AV,,' (= R T d In q/dP) is the activation volume of viscous flow. At a sufficiently high viscosity region where B becomes relatively small as compared with $, eq 12 reduces to

AV = AVi* + AV,*

Summary The IE formation in DPP is retarded as pressure is applied. This experimental data are treated as the dependence of solvent viscosity and are best interpreted in terms of the hindered rotation model originally based on Kramers' equation, in which a slip boundary condition should be adopted for the specification of the friction coefficient. The observed activation volume, A P , is made up of two contributions: (a) the activation volume of viscous flow, At',*, and (b) the intrinsic activation of IE formation, AV,'.

(13)

The differential coefficient of In (c&/I#Q) against pressure was almost constant, and then the value of AV was calculated to be 24.8 cm3/mol. Further, by fitting the viscosity change with pressure to polynomials, we calculated AV,* to be 27.3 cm3/mol at 2.5 kbar. As a result, AV,' = -2.5 cm3/mol is obtained. We also calculated AV,* in terms of eq 12 by using the value of B obtained above: all values of A?',' result in an analogous value at pressures greater than 1 kbar. At the lower pressure, however, AV,' deviates from this value, since the viscosity dependence on pressure is not so accurately determined near 1 bar. Taking into account the suppositions included, it may be difficult to discuss this value in more detail. But it is concluded at least that the intrinsic volume change of activation is quite small, if any. The volume change proposed in intermolecular excimer

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Acknowledgment. This research was supported in part by a Grant-in-Aid for Scientific Research No. 59540262 from the Ministry of Education, Science, and Culture. Registry No. DPP, 61549-24-4;TMPD, 1921-70-6. (22) Forster, Th.; Leiber, C. 0.;Seidel, H. P.; Welter, A. Z. Phys. Chem. (Munich) 1963, 39, 265.

Electron Paramagnetic Resonance and Infrared Spectra of Ga(CO), in Hydrocarbon Matrices' J. A. Howard,* R. Sutcliffe,' Division of Chemistry, National Research Council of Canada, Ottawa, Canada K1A OR9

C. A. Hampson, and B. Mile* Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L3 3AF (Received: February 12, 1986)

Ga(C0)2 has been prepared in adamantane and cyclohexane at 77 K from Ga atoms and CO and its EPR and IR spectra have been examined and together show the molecule to be bent with a 2Bl ground state in C, symmetry. Powder EPR spectra in cyclohexane at 10 K yielded the magnetic parameters: aI1(69)= 275.3 MHz, aI1(71)= 351.5 MHz, a,(69) = 8.3 MHz, 18 MHz, g, = 2.0010, gYu= 2.0120, g,, = 1.985, Q(69) = 8.4 MHz, ~ ~ ( 7 =1 10.5 ) MHz, u,,(l3) 2.8 MHz, a,(13) and Q(71) = 6.6 MHz. Magnetic parameters in adamantane at 230 K were almost isotropic with aGa 60 MHz and g = 1.9965. Line widths were, however, too broad to resolve differences in the 69Gaand 'IGa hyperfine interactions and a I3Chyperfine interaction. Isotropic spectra of 69Ga(CO), and 69Ga('3C0)2have been obtained from isotopically pure 69Ga and CO in adamantane at 230 K and gave the isotropic parameters a(69) = 62 MHz and g = 1.9961, but the lines were too broad to give an isotropic 13C hyperfine interaction. Magnetic data are consistent with a bent planar a radical having the unpaired electron in a 2B, orbital constructed from the gallium 4p, and carbon monoxide 2a,* orbitals. The IR spectrum of Ga(C0)2 in adamantane has a symmetric CO stretching mode at 2009 cm-' and an antisymmetric CO stretching mode at 1929.9 cm-I, consistent with a bent dicarbonyl. The relative intensities of these bands gave a CGaC angle of 120° and the frequencies gave the force and interaction constants kco = 15.67 and kco.co = 0.63 mdyn/& respectively.

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Introduction Kasai and Jones3 have recently reported powder electron paramagnetic resonance (EPR) spectra of dicarbonylmonogallium(O), Ga(C0)2, produced in solid argon by reaction of the metal atom with carbon monoxide at -4 K. From these spectra it was concluded that this molecule is bent at the CGaC linkage and that the unpaired electron is located in a molecular orbital constructed from the metal 4p, orbital and the 2a,* orbitals of (1) Issued as NRCC No. 25564. (2) NRCC Research Associate 1979-1984. Present address: Biotechnology and Chemistry Department, Forintek Canada Corp., Ottawa, Canada. (3) Kasai, P. H.; Jones, P. M. J. Phys. Chem. 1985, 89, 2019-2021.

0022-3654/86/2090-4268$01.50/0

the ligands. There is a brief mention of an infrared (IR) spectroscopic study of the reaction of Ga atoms with CO in solid krypton by OgdenS4 Symmetric and antisymmetric CO stretching modes at 2006 and 1912 cm-' were assigned to a gallium dicarbonyl, Ga,(CO), of unknown gallium stoichiometry, but no spectra have been published and there has been no further discussion or work on the vibrational spectra of Ga/CO/inert matrix codeposits. In the present paper we report a combined EPR and I R study of the reaction of Ga atoms with C O in solid adamantane and (4) Ogden, J. S. In Cryochemistry; Moskovits, M., Ozin, G. A., Eds.: Wiley: New York, 1976; p 247.

Published 1986 American Chemical Society

The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4269

EPR and IR Spectra of G a ( C 0 ) 2

9146.5 MHz

TO DETECTOR

FROM SOURCE

Figure 1. Modification of the optical path of the FTIR spectrometer.

cyclohexane on a rotating cryostat at 77 K that together show conclusively that Ga(CO)* is the major gallium carbonyl produced a t cryogenic temperatures and it is “visible” to both EPR and IR spectroscopy; i.e., it is not a linear degenerate doublet n species. Furthermore there is tentative evidence for a gallium monocarbonyl in IR spectra of Ga/CO/hydrmrbon deposits, although such a species has not been detected by EPR spectroscopy.

Experimental Section The rotating cryostat and furnace used to vaporize gallium have In the present work Ga atoms were been described previou~ly.~-~ generated from a resistively heated tungsten basket (no. 12070, Ernest F. Fullman, Inc., Schenactady, NY) at 1000 OC. Low concentrations of atoms were deposited onto the surface of solid adamantane or cyclohexane at 77 K and bombarded with a stream of CO. The resulting carbonyl was then covered and isolated in the next layer of hydrocarbon matrix deposited in ten- to a hundredfold excess. Infrared spectra of deposits on the drum were obtained by using a Mattson Sirius 100 FTIR spectrophotometer; the optical path of the infrared beam of which had been modified as indicated in Figure 1. In the modification the beam was deflected to a focus on the drum surface by two plane mirrors A and B and a spherical mirror C (f = 40.6 cm). The reflected beam, which had passed through the deposit, was collected and collimated with a spherical mirror D (f = 40.6 cm) and diverted to a cooled MCT detector by two plane mirrors E and F. In a typical run 256 background spectra from the uncovered drum surface were averaged before averaged spectra of deposits of CO/matrix and Ga/CO/matrix were Obtained Over the range 4000-500 cm-’ at 4-cm-’ resolution. Spectra of the G a / C O reaction were then obtained by subtracting the from the Ga/Co/matrix the intense co stretching bands in most of the carbonyls were clearly visible in unsubtracted spectra. After the infrared Of the deposit had Obtained at 77 K, a small sample was removed from the cryostat and examined by EPR spectroscopy in either a Varian E-4 Or E-9

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( 5 ) Bennett, J. E.; Thomas, A. Proc. R. SOC. London, A 1964, 280, 123-1 38. ( 6 ) Bennett, J. E.; Mile, B.; Thomas, A.; Ward, B. Adv. Phys. Org. Chem.

(7) Buck, A. J.; Mile, B.; Howard, J. A. J. Am. Chem. SOC.1983, 205, 3381-3387.

Figure 2. Powder EPR spectrum of Ga(CO), in cyclohexane at 77 K (a) and at 4 K (b). Spectrum simulated from the magnetic parameters given in the text (c).

spectrometer at temperatures from -4 to 300 K. The microwave frequency of the EPR spectrometer was measured with a Systron-Donner Model 6057 frequency counter, and the magnetic field of the magnet was measured with a Varian E-500 N M R gaussmeter. The deposit that remained on the drum was annealed by flushing out the liquid nitrogen from the drum, but not from the outer heat shield, by passing air at room temperature through the drum. With this technique the temperature of the deposit increased slowly and could be maintained at a certain temperature simply by interrupting the passage of air. IR spectra were recorded at selected temperatures during this warmup. High-purity gallium was provided by Dr. C. M. Hurd (NRC, Ottawa, Canada). Isotopically pure 69Ga (99.46 atom W ) was prepared by electrolysis of 69Ga203(Oak Ridge National Laboratory, T N ) in 10% sodium hydroxide.8 Adamantane and cyclohexane were obtained from Aldrich, natural CO was obtained from Matheson, and I3CO (99.8 atom W I3C) and perdeuteriocyclohexane were obtained from Merck, Sharpe and Dohme, Canada Ltd.

Results EPR. Gallium atoms and natural C O in cyclohexane at 77 K gave an orange deposit that exhibited the EPR spectrum shown in Figure 2a. This spectrum consists of two sets of four equally spaced absorption shaped lines centered at gxx = 2.0010 and two weak features centered at gyy = 2.0120 and g,, = 1.9850. The to the parallel transitions of the powder two quartets are spectrum of a species containing one gallium atom and arise because there are two naturally abundant Ga isotopes with I = 3/2, 69Ga (natural abundance = 60.5%) and ’IGa (natural abundance = 39.6%). The Ga hvDerfine interactions were measured as one-third of the field increment between the M I = f 3 / 2 resonances and gave all(69)= 275.3 MHz and all(71)= 351.5 MHz. The ratio of these hyperfine interactions is equal to the (8) Hampel, C. A. The Encyclopedia ofElectrmhernisfry; Reinhold: New York, 1964; p 600.

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The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 8973.9 MHz n

Howard et al.

20G 9153.1 MHz

3250 G Figure 4. EPR spectrum of Ga(CO), in adamantane at 77 K (a) and at 230 K (b).

Figure 3. Central features of the spectrum of Ga('3C0)2 in perdeuteriocyclohexane at 77 K (a). Simulated spectrum using the parameters given in the text (b).

ratio of the magnetic moments of gallium, ~ ( 7 1 ) / p ( 6 9 )= 1.27. The perpendicular features of this spectrum are weak, from which we can conclude that a,,(Ga) >> a,(Ga). The central features sharpened considerably on cooling the sample to 10 K (Figure 2b), giving two groups of lines that are almost identical in appearance to the group of lines observed in the center of the powder spectrum of C U ( C O )in ~ argong and hydrocarbon matrices.I0 This group of lines in the case of C U ( C O ) is ~ a narrow closely spaced triplet of anomalous phase and is ascribed to forbidden transitions. These arise because the hyperfine constant for a given orientation, a, the anisotropy of the hyperfine tensor a,,- a,, and the nuclear Zeeman term, g,,@,,H,are all of similar magnitudes. The gallium carbonyl powder spectrum can be simulated" (Figure IC) by using 1 ) the following parameters: a,,(69) = 275.3 MHz, ~ ~ ~ (=7 351.5 MHz, a,(69) = a,,(69) = 8.3 MHz, aJ71) = a,,(71) = 10.5 MHz, g, = 2.0010, gyy= 2.0120, g,, = 1.985, Q(69) = 8.4 MHz, and Q(71) = 6.6 MHz. The shapes of the central features were quite sensitive to the magnitudes of avv and arr and could not be simulated with values of 5 M H z larger or smaller. Furthermore quadrupole terms were essential for a reasonable simulation. Ga atoms and I3CO in perdeuteriocyclohexane at 77 K gave a powder spectrum similar to Figure 2a except that there was evidence for I3C hyperfine interactions in t h e y and z directions and line broadening of the x components. As the sample cooled to 10 K the spectrum narrowed to give the central features shown in Figure 3a. This spectrum can be simulated (Figure 3b) if we assume that the species responsible for the spectrum contains one gallium atom and two equivalent carbon nuclei and by using the gallium hyperfine interactions and g factors given above and ayv( 13) = a,,(13) = 18 M H z and a,,(13) = 2.8 MHz, i.e., with an axially symmetric I3C hyperfine tensor. Ga atoms and natural CO in adamantane at 77 K gave a spectrum (Figure 4a) consisting of two quartets that are assigned to the parallel features of a powder spectrum with ~ ~ ~ ( =6 210 9 ) MHz, a,,(71) = 267 MHz, and g, = 1.9993. These parallel features are broader and a,,values are smaller than in cyclohexane. The forbidden transitions are narrower in adamantane at 77 K than they are in cyclohexane a t this temperature and occur at a field corresponding to the center of the parallel features. This gw g,, and that the molecule suggests that in adamantane g, is rocking about the y axis. The spectrum narrowed and ax,

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(9) Kasai, P. H.; Jones, P. M. J. Am. Chem. SOC.1985, 107,813-818. (10) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. J . Phys. Chem. 1986, 90, 1033-1036. ( 1 1) Belford, R. L.; Nilges, M. J. EPR Symposium, 21st Rocky Mountain Conference, Denver, CO, Aug, 1979.

a

b

' I

32190

Figure 5. EPR spectrum of Ga('3C0)2in adamantane at 77 K (a) and at 230 K (b).

increased on cooling to 10 K until it had the appearance of Figure 2a; i.e., it was identical with the spectrum in cyclohexane at 77 K. At this temperature a,(69) = 268.7 MHz and g,, = 2.0010. The spectrum did not change after several hours at 10 K; Le., it did not take on the appearance of Figure 2b. As the sample warmed, the spectrum gradually became more isotopic until at 230 K the almost isotopic quartet shown in Figure 4b was obtained. The lines of this spectrum were too broad (AHpp 10 G ) to resolve the two quartets from the gallium isotopes. The average gallium hyperfine interaction was measured as 61.5 M H z and g = 1.9965. Samples of isotopically pure 69Ga(C0)2in adamantane were prepared in the hope that solution-like spectra of the quality that we have previously obtained A1(C0)212and C U ( C O ) , ' ~ Jwould ~ be produced in the temperature range 2OC-250 K. Unfortunately

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(12) Chenier, J. H. B.; Hampson, C. A.; Howard, J. A,; Mile, B.; Sutcliffe, R. J . Phys. Chem. 1986, 90, 1524-1528. (13) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. Chem. Phys. Leu. 1985, 117,115-117.

The Journal of Physical Chemistry, Vol, 90, No. 18, 1986 4271

EPR and IR Spectra of G a ( C 0 ) 2 0.323

a 0.204 A A

and a doublet B at 1833.5and 1760 crn-l. The species responsible for these spectra must, therefore, contain two and one carbonyl ligands, respectively. Bands from C were not observed either because they were too weak or were masked by bands A. It is noteworthy that there is a considerable enhancement in the intensity of the C O stretching modes in Ga/CO deposits over the free C O in adamantane, as has been found for most metal carbonyls. 14,15 Annealing experiments revealed that the relative intensity of bands A remained constant and disappeared at the same rate as the temperature was increased, which indicates that they are associated with the same species. Band B disappeared before bands A and the relative intensity of B to A decreased as the C O pressure was increased. The presence of two infrared-active C O stretching modes in the IR spectrum of the gallium dicarbonyl is consistent with a bent molecule, and the higher frequency band is assigned to the symmetric C O stretching mode and the lower frequency band to the antisymmetric CO stretching mode.I6 The gallium dicarbonyl responsible for this IR spectrum is almost certainly the same one that is responsible for the major transitions in the EPR spectrum, viz., Ga(CO)> The two bands labeled A' in Figure 6b are assigned to Ga(13C0)2in a different trapping site. Assignment of the other two bands at 1833.5 and 1970 cm-I is more difficult. There are three possibilities for the band at 1833.5cm-l: (i) a monocarbonyl, GaCO, because bent carbonyls such as CH3C0" and HC0I8have C O stretching frequencies of 1796 and 1861 cm-l, respectively, (ii) a doubly bridged carbonyl, Ga2C0, and (iii) a triply bridged carbonyl, Ga3C0. It is unlikely to be the monocarbonyl because there was no evidence for a bent monocarbonyl in the EPR spectra of Ga/CO deposits that would have a characteristic 13C hyperfine interaction of -120 G.I9 There was a broad band in the IR spectra from Al/CO/ adamantane deposits20 at 1710 cm-' that has been tentatively assigned to the cluster carbonyl A13C0. Ga3 is not so readily formed or observed as Al?' in hydrocarbon deposits on the rotating cryostat, though it has a similar structure.22 Furthermore the frequency of 1833.5 cm-' is -120 cm-' higher than that for A13C0. We are, therefore reluctant to assign this band to Ga3C0. It is, however, in the range expected for doubly bridging p2-C023 and is tentatively assigned to Ga2C0, which would probably have a singlet ground state and would not be visible by EPR spectroscopy. The weak band C is close to the value expected for the linear monocarbonyl GaCO. This species has a 211112electronic ground state, and unless the degeneracy of the two p orbitals is lifted by the matrix it will not be detected by EPR spectroscopy.

0.08

-0.033

0.088

I

1

A

C

' 2doo ' I&

' lalW '

I -0.033t

'

2;w

o!o

WAVENUMBERS/C~-~

Figure 6. IR spectra given by Ga/CO (a), Ga/13C0 (b), and Ga/ C O / 1 3 C 0 (c) in adamantane at 77 K.

although a weak isotopic spectrum was obtained with a(69) = 62 M H z and g = 1.9961,the transitions were broad. Furthermore the spectrum was masked by a broad anisotropic feature that ruined the quality of the spectrum. Reaction of G a atoms with I3COin adamantane at 77 K gave the spectrum shown in Figure 5a. The parallel features of this spectrum are broader than those from G a and natural CO and indicate that a,,(13) 2.8 MHz. The narrow central features suggest two equivalent carbon nuclei and can be simulated with aw = a, = 18 MHz. As the sample warmed to 240 K, two p r l y resolved quartets were apparent with a(69) 60 M H z and 471) 70 M H z . A sample of 69Ga(13C0)2was prepared in adamantane from isotopically pure 69Gaand I3CO. At 235 K the spectrum consisted of four broad lines with a(69) = 62 M H z and g = 1.9961. The lines were, however, too wide to give a resolved 13Chyperfine interaction. ZR. IR spectra given by Ga atoms and natural CO, I3CO, and a 1:l mixture of CO and 13C0in adamantane at 77 K in the C O stretching region (2200-1700 cm-l) after subtraction of the adamantane spectrum are shown in Figure 6. Natural C O gave two sharp bands, A, at 2009 and 1929 cm-', a broad band, B, at 1833.5 cm-l, and a weak band at 1970 cm-I. Exactly the same pattern was obtained with I3€O with all the bands shifted to lower frequencies, showing that they are all associated with gallium carbonyls. Thus bands A were at 1966.6 and 1887.5 cm-', band B was at 1794.9cm-', and band C was at 1940 cm-'. In addition two sharp bands A', with the same relative intensity as bands A but a t lower frequency (1936 and 1856.6 cm-I), were observed in the Ga/I3CO system. The spectrum from G a / C 0 / I 3 C O gave two triplets, A, with bands at 2009, 1993.6,1966.6,1929.9, 1902.9 and 1887.5 cm-' N

N

N

Discussion EPR. It is apparent from EPR spectroscopy that dicarbonylgallium, Ga(C0)2, is the major paramagnetic product from the reaction of Ga atoms with C O in hydrocarbon matrices on a rotating surface at 77 K. This dicarbonyl is also formed at 4 K in argon on a stationary ~ u r f a c e and , ~ it has the same stoichiometry as the major paramagnetic product from the reaction of A1 atoms with C O at cryogenic temperatures, Al(CO)2.12924 (14) Samvelyan, S. Kh.; Aleksanyan, V. T.; Lokshin, B. V. J . Mol. Spectrosc. 1973, 48, 47-56. (15) Bigorgne, M. Spectrochim. Acta, Part A 1976, 32A, 673-678. (16) Cotton, F. A.; Wilkinson, G. Aduanced Inorganic Chemistry; Interscience: New York, 1972; p 701. (17) Bennett, J. E.; Graham, S. C.; Mile, B. Spectrochim. Acta, Part A 1973, 29A, 375-383. (18) Milligan, D.E.; Jacox, M. E. J. Chem. Phys. 1964, 41, 3032-3036. (19) Fischer, H.; Paul, H. Landolt-Bornstein, New Series; Fischer, H.; Hellwege, K.-H., Eds.; Springer-Verlag: West Berlin, 1979; Vol. 9, Part b, pp 318-324. (20) Chenier, J. H. B.; Hampson, C. A,; Howard, J. A,; Mile, B. J. Chem. SOC..Chem. Commun.1986. 730-732. (21) Howard, J. A.; Sutcliffe, R.; Tse, J. S.; Dahmane, H.; Mile, B. J . Phys. Chem. 1985,89, 3595-3598. (22) Howard, J. A.; Mile, B., unpublished result. (23) Reference 16, p 692. (24) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1984,106,8018-8020.

4272 The Journal of Physical Chemistry, Vo1. 90, No. 18, 1986

Howard et al.

TABLE I: Anisotropic and Isotopic Magnetic Parameters of 69Ga(C0)2"

matrix temp/K a,(69) argonb 4 44.5c cyclohexane 10 8.3 adamantane 10 adamantane 77 adamantane 230 "In MHz. bReference 3. > a,, where a, = (a,,, a,,)/2. This gives spectra with prominent and well-resolved parallel features and weak and poorly resolved perpendicular features a t 77 K. The value of a,(Ga) is significantly smaller in cyclohexane (-8 MHz) than it is in argon (44.5 MHz), while the value of all is not too different. Adamantane proved to be a poor matrix for the powder spectrum of Ga(C0)2 because of some residual tumbling motion of the molecule. Despite this solution-like spectra of the quality obtained for A1(C0)2, Al('3CO)2, and A1(C170)212 were not obtained at higher temperatures (210-250 K). The measured isotropic hyperfine interaction of 62 M H z is smaller than the value of 97.3 MHz calculated from all 2a,/3, assuming that aL is positive. The agreement is not much better (86.2 MHz) if a, is assumed to be negative. We can, however, conclude that the unpaired spin population in the gallium 4s orbital, P(4S)Gaj is negligable because the atomic parameter A for 69Ga is 12210 M H z . ~ ~The unpaired electron on gallium, therefore, resides in the 4p, orbital, and the small unpaired s spin population must arise by core polarization. The unpaired spin population in the 4px orbital, p(4p,)G,, is estimated M H z dividing the dipolar tensor (Adip) by aP,where P, the atomic parameter for unit spin population in the gallium 4p orbital, = 509.6 MHz and a,the angular factor, = 0.4.25Adip is obtained from the empirical parameters all and a, by using the relationship

~~

R.,Preston, K F J . Magn. Reson

1978, 30, 557-582

e 110

0.44

0.09

112 110

0.55

0.09

0.44

0.06

0

120 120

ref 24 28 12 12 3 4

this work this work

I

% -5-

;

4P

w

4s

5-10-

-15

-

5a

-*'I

+

( 2 5 ) Morton, J.

Pc 0.09

5 t

+

Because a, is small and of unknown sign in cyclohexane we conclude that Adlp= q / 3 = 91.8 MHz and p(4pJ0, = 0.45. This is smaller than the value of 0.55 reported by Kasai and Jones3 in argon. The larger value of p(4Px)Ga requires a substantially larger and negative value of a,(Ga), and we could not simulate our lowest temperature spectrum in cyclohexane by using Kasai and Jones' value of a,. Turning to the I3C hyperfine interactions, the perpendicular value of 18 MHz must be negative because all(13) < al( 13) and Adipis always positive (eq 1). If ~ ~ ~ ( is1 +2.8 3 ) MHz, Adip = 6.9 MHz, and if it is -2.8 MHz, Adip = 5.1 MHz. Dividing these values by 0.4P, where P = 268.5 M H z for 13C,25gives p(2pJC = 0.06 and 0.05, values that are somewhat smaller than that of 0.09 calculated by Kasai and Jones.3

Phl

0.42

Go

co

Figure 7. Qualitative molecular orbital energy level diagram for Ga(C-

Oh.

The total unpaired spin count for Ga(C0)2, ignoring the negligible unpaired s spin populations, is given by c p

= P(4Px)Ga + 2d2px)C + 2P(2Px)0 = 0.45

+ 0.12 + 2p(2px)o

If p(2pJO is similar to the value of 0.1 1 in A1(C0)2,12Cp = 0.79 so that we can account for -80% of the unpaired spin. Unlike the hyperfine tensor the g tensor is certainly orthorhombic with one value close to the free spin value of 2.0023, one below free spin (Ag = -0.017), and one above free spin (Ag = 0.0097). These g shifts can best be rationalized with reference to the energy level scheme shown in Figure 7. The positive g shift must arise because mixing of the half-filled bl orbital with the filled a l orbital is more important than mixing with the empty a l orbital. Both give a bl representation and g shifts in the y direction. Mixing of the bl orbital with the empty b, orbital gives an a2 representation and a negative g shift in the z direction. Mixing between the bl orbital and the empty a2 to give a b2 representation must be weak and predicts a very small negative g shift when Hois parallel to x . Kasai and Jones' have concluded that Ga(C0)2 is isostructural with AI(C0)2;i.e., it is a bent planar K radical with C, symmetry in which the valence s and p orbitals have undergone sp2 hybridization. ESR evidence alone does not give the C-Ga-C angle 0 and does not eliminate a structure with 8 = 90' snd no valence orbital hydrization. It does, however, eliminate a linear dicarbonyl with 8 = 180' because this is a 2111/2species and would be ESR silent.

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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

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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 = [email protected] - 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.

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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 . Am. 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