J . Phys.
1524
C'httti.
cm-l, which was due to C H D 2 F molecule. It is concluded from its intensity that the concentration of this species does not exceed 1 %. Many of the binary transition bands fall into the C D stretching fundamental region. Very complicated interactions are expected among the bands just like the ones in CH,F molecule. The band intensities of 2v2, ( v 2 + v 5 ) , ( v 2 + v,), ( v 3 + us), and (v2 + v6) could not be determined because they are underlying the much stronger vl, vj, and 2v50, whereas the intensity of 2 1 was ~ ~ obtained by simply integrating the appropriate absorption area. We could not find (v5
+ v6).
The ( v 3 + v6) band was found a t 1898.97 cm-I. A null absorption was given to both components of the 2v6 band. The observed band intensities for CD3F are summarized in Table IV. E. Experimental Second Dericatices of Dipole Moment. The observed and predicted intensities are compared in Tables 111 and IV. If the observed intensities are compared with the pred I values, agreement is not good a t all. Definitely, contribution from the electrical anharmonicity is needed to explain the observed intensities. In fact, agreement with pred I1 values is much better than for pred I. For most bands, the pred 11intensities agree with the observed ones within a factor of 2 or so, indicating that the a b initio P," values are pretty reliable. Discrepancy is only noted for the 2uS0band; in order to explain the intensity of this band satisfactorily, the complicated interactions must indeed be taken into accoutit precisely as was done by Champion et al. to illustrate the rotational fine structure of the spectra.' Tables V and V I show the result of analysis of the transition dipole moments. In these tables, the column I shows the observed values of R,," as defined in eq 7-13. Their signs were chosen as follows; at first, the contribution of the mechanical anharmonicity
1986, 90, 1524-1528 to R,," was calculated as shown in column 11. Then, the values of P,,were obtained by subtracting the numbers listed in column 11 from those in column I, and are shown in column 111. The resulting values of P,, for the two possible signs of Rr," were compared with the a b initio result listed in column IV. The signs were so chosen that the P,, thus obtained may agree better with the a b initio value. If they are chosen in this way, agreement between the observed and calculated P,,is very good. The numbers in parentheses in column 111 are for the alternative signs of the transition moment. In a previous paper, we have analyzed the effective dipole moment in the excited vibrational state^.^ It was found that P3,H = -0.0148, P,jD = -0.0195, P5sD= 0.0043, and P6GD= -0.0101 D. These are to be compared with the present results, Le., -0.01 52, -0.0304, 0.3885, and -0.0122 D. Agreement between the two results is excellent for P,,Hand p(,GD,while it is not good for P,," and PS5D.Probably, the discrepancy for P,,D is due to an error involved in the 2v3 band intensity. It is possible that the ( u s v 6 ) bands are underlying 2u3 with certain intensities (e.g., see pred I1 in Table IV). As the value of PI33 is small (1.6 cm-I, ref 4), this band cannot be much affected by Fermi resonance with v , . On the other hand, the result for PssDis rather natural because we did not consider the effect of close resonances occurring in the 2u5 region in the present analysis. In conclusion, the agreement obtained for P33H and P66D in the above may indicate the degree of accuracy of P,," resulting from the analysis of the binary transition intensities and/or of the effective dipole moments by using the anharmonic force field employed here.
+
Registry No. CH,F, 593-53-3; CD,F, 1 I 1 1-89-3
EPR Spectra of AI(CO)* in Hydrocarbon Matrices' J. H. B. Chenier, C. A. Hampson,* J. A. Howard,* B. Mile,* and R. Sutcliffe3 Division of Chemistry, National Research Council, Ottawa, Ontario, Canada K l A OR9 (Received: July 31, 1985; In Final Form: October 9, 1985)
AI(C0)2 has been prepared in inert hydrocarbon matrices from AI atoms and CO and its EPR spectrum examined. Powder spectra in cyclohexane and adamantane indicate that Al(CO), is an almost axially symmetric species with the magnetic parameters Al,(AI) = 140 MHz, A,(Al) = 44.8 MHz, A,l(C)< 2.8 MHz, A , ( C ) = 25 MHz, gl,= g, = 2.0020. Isotropic spectra in adamantane at 2.50 K give A(AI) = 72.6 MHz, A(C) = -15.4 MHz, A(0) = -12.2 MHz. These data are consistent with a bent planar r radical of C, symmetry having the unpaired electron in a molecular orbital perpendicular to the molecular plane and constructed from the aluminum 3p, and carbon monoxide 2r,* orbitals while the aluminum "lone-pair" electrons reside in a sp2 orbital directed along the C2 axis.
Introduction Kasai and Jones4 have recently published the powder EPR spectrum of aluminum dicarbonyl produced by co-condensing AI atoms and CO in solid argon a t 4 K. This spectrum provided evidence that AI atoms and CO reacted under these conditions to give a paramagnetic carbonyl containing one aluminum atom and two carbon monoxide ligands, Le., Al(CO),. Although there was no doubt from the powder spectrum that the aluminum carbonyl did contain one aluminum nucleus the number of carbon monoxide ligands had to be determined by a comparison of ex-
perimental and simulated spectra of AI/l3CO codeposits. A further difficulty with analysis of the powder spectrum involved determining the signs of the anisotropic aluminum hyperfine interactions. We have recently demonstrated5-' that metal atom carbonyls can be prepared in a rotating cryostat by sequential deposition of metal atoms and carbon monoxide in inert hydrocarbon matrices. An important advantage of hydrocarbon over rare-gas matrices is that they can be warmed to much higher temperatures without loss of the paramagnetic transient. This together with the larger size of the substitutional cavity (5-6 A) results in
( 1 ) Issued as NRCC No. 25378.
(2) Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L3 3AF. (3) NRCC Research Associate 1979-1 984. Present address: Biotechnology and Chemistry Department, Forintek Canada Corp., 800 Montreal Road, Ottawa, Canada. (4) Kasai, P. H.: Jones, P. M. J . A m . Chem. SOC.1984, 106, 8018-8020.
0022-36S4/86/2090- I S24$0 1.50/0
(5) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. Chem. Phys. Lett. 1985, I 1 7, 1 1 5-1 17. (6) Howard, J. A.; Mile, B.; Morton, J . R.; Preston, K. F.; Sutcliffe, R. J . Phys. Chem. 1985, 90, 1033-1036. (7) Hampson, C. A,; Howard, J. A,; Mile, B. J . Chem. SOC.,Chem. Commun. 1985, 966-967.
Published 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 8. 1986 1525
EPR Spectra of A1(C0)2
I
1 ~
All
a
y 3262.8G l 9124.9MHz
3260 G
I'
C
Figure 1. The powder EPR spectrum of AI(CO), (a) and Ai(l3CO), (b) in cyclohexane at 77 K and the computer-simulated spectrum of AI("CO), in cyclohexane (c).
isotropic spectra being observed. Application of this technique to the Al/CO system has enabled us to obtain both the powder and isotropic spectra of AI(CO),. This work using I3CO and C170 give unequivocally the stoichiometry of the reaction, the signs of the aluminum and carbon anisotropic hyperfine interactions, and the first I7O hyperfine interaction in any metal carbonyl compound.
Experimental Section The rotating cryostat and instruments used to record and calibrate EPR spectra have been described previously.8 Aluminum was evaporated from a tungsten basket (No. 12070, Ernest F. Fullam, Inc., Schenectady, NY) suspended from the molybdenum electrodes of the furnace. High-purity aluminum was provided by Dr. C. M. Hurd (NRCC, Ottawa). Adamantane and cyclohexane were obtained from Aldrich, I2CO from Matheson, I3CO (99.8 atom % I3C) from Merck Sharpe and Dohme, Canada Ltd., and CI7O (36.8 atom % I7O) from Prochem Isotopes, Summit, NJ 07901. Results The EPR spectrum given by AI atoms and natural C O in cyclohexane a t 77 K is shown in Figure l a . It is not as well resolved as Kasai and Jones' spectrum of A1(CO)2 in argon at 4 K4 and consists principally of two sextets which are assigned to the parallel and perpendicular hyperfine interactions from an almost axial species containing one aluminum nucleus ( I = 5/2). Analysis of the spectrum yielded the following parameters: IAll(27Al)l= 140 MHz, IA,(27Al)l = 44.8 MHz, gll = g, = 2.0020. A computer-simulated spectrum based on these parameters gave a reasonable fit with the experimental ~ p e c t r u m . ~ The spectrum given by AI atoms and I3CO in C 6 H l zis shown in Figure lb. The parallel features are broader than those from l2CO while the perpendicular transitions show poorly resolved triplets with a I3C spacing of -25 MHz. This suggests that the carrier of the spectrum is AI(CO),. A computer simulation of the spectrum of A1(I3C0), using the parameters quoted above = 3 M H z is shown in Figure IC. and IAl,(13C)( Reaction of A1 atoms and C O adamantane a t 77 K gave the spectrum shown in Figure 2a. The parallel features are broader and the perpendicular features are sharper than the corresponding features in cyclohexane. The spectrum can be analyzed in terms (8) Buck, A. J.; Howard, J. A.; Mile, B. J . Am. Chem. SOC.1983, 105, 3 3 8 1-3387. (9) Lefebvre, R.; Maruani, J. J . Chem. Phys. 1965, 42, 1480-1496.
Figure 2. The powder spectrum of AI(C0)2 in adamantane at 77 K (a) and the solution spectrum of AI(CO), in adamantane at 243 K.
of two sextets with IAll(27A1)1= 108.7 MHz, IA,(27A1)1 = 54.6 MHz, and gll= gl = 2.0021. At this temperature All is smaller and A , is larger than the corresponding values in cyclohexane. The spectrum in adamantane slowly changed on cooling to -4 K (over several hours) at which temperature the parallel transitions sharpened considerably and All(27A1)increased gradually to 140 MHz and A,(27Al) decreased to 44.8 MHz, values identical with those in cyclohexane at 77 K. The slow rate a t which the lines sharpened is probably a measure of the rate of reorganization of the matrix to the most stable low temperature form and indeed the rate of such a phase change can be determined by using the sharpening of the EPR transitions as a monitor. When this sample of A1(C0)2 was gradually warmed in the cavity of the spectrometer the spectrum became progressively more isotropic until at 243 K the sextet shown in Figure 2b was obtained. This spectrum yielded the isotropic parameters A,,,(27AI) = 72.6 M H z and g = 2.0021. Reaction of A1 atoms with I3CO in adamantane at 77 K gave the powder spectrum shown in Figure 3a. The parallel features are again broader than those from natural C O and the perpendicular transitions resolved into triplets with lAl(13C)l 25 MHz. When this sample was warmed in the cavity of the spectrometer the spectrum again became progressively isotropic until at 243 K a sextet of triplets (Figure 3b) was obtained. This proves conclusively that the carrier of the spectrum has one aluminum atom and two carbon monoxide ligands. This spectrum gave the isotropic parameters A(27A1)= 72.3 MHz, A(I3C)= 15.4 MHz, and g = 2.0020. A1 atoms react with I70-enriched CO in adamantane a t 77 K to give a powder spectrum similar to the one shown in Figure 2a, Le., I7O hyperfine interactions were not resolved. The solution spectrum obtained upon warming to 237 K did, however, show the presence of I70-enriched aluminum dicarbonyl and is presented in Figure 4a. The sextet is assigned to A1(12C'60)2and the sextet of sextets, the lowest and highest field sextets of which are indicated by the stick diagram in the figure, to A1('2C170)(12C160). There was no indication of transitions that could be assigned to the doubly enriched species Al(C170)2in this single-scan spectrum. The oxygen hyperfine was isotropic and A(I7O) = 12.2 MHz. Signal averaging of the spectrum improved the signal-to-noise ratio considerably and the outer lines ( M I = 1 5 and 1 4 ) of the
-
1526
The Journal of Physical Chemistry, Vol. 90, No. 8 , 1986
Chenier et al.
TABLE I: Anisotropic Magnetic Parameters of AI(CO)," C
AI
matrix
temp/K
A,,
A,,
Ax,
argonb cyclohexane adamantane
4
39.4
77
44.8 44.8 54.6
38.7 44.8 44.8 54.6
145.7 I40 I40 108.7
4 77
adamantane
A I* 252 25 25 25
A,, 42
A, 252 25 25 25 ).
gir 20043 2.0020 2.0020 2.0021
gzz
19990 2.0020 2.0020
AI1(13),one finds it must be negative with a value of -15.4 MHz. Dividing this value by A for carbon of 3777 MHz" gives a s spin population ( ~ ( 2 s )=~-0.004. ) Substitution of the measured values of Ai,(13) = -15.4 M H z and A,(13) = -25 M H z into the expression Ai, = (All+ 2A,)/3 gives All = , 3 . 8 MHz, close to the value estimated from the increase in line width of the powder spectrum when ' I C 0 was used instead of natural CO. The anisotropic component of the "C hyperfine tensor, Adip. is therefore (3.8 + 15.4)/2 = 9.6 MHz which divided by 2P/5 (where P for carbon = 268.5 MHz") gives a positive unpaired p spin population of 0.09 in the carbon px orbital. The isotropic oxygen hyperfine interaction of 4.35 G (12.2 M H z ) does not enable us to calculate A,, for oxygen and hence (14) Morton. J. R.; Preston, K. F. 3. M o p . Reson. 1978. 30. 557-582.
-
the 9 I IOD. The bonding is believed to involve donation of the electrons from the 5 0 orbitals of the CO's into two empty sp2hybridized orbitals on aluminum, the third sp2-hybridized orbital containing a lone pair of electrons, and backdonation of the unpaired electron in the 3px orbital into the 2r,* orbitals of the COS. This would mean that AI(CO), is a planar T species belonging to the representation in C, symmetry where the z axis is the Czaxis. AI(CO), can,therefore, be considered to be isolobal with dimethylaminyl, (CH,),N. An INDO molecular orbital calculation' on AI(CO), using the structural parameters 0 = I IO', r(AI-C) = I .SO A, and r ( C 4 ) = 1.15 A provides support for structure 1 because it gives the following spin population distribution: p(3s),, = 0.02, ~ ( 3 p ~ ) , ~ = 0.42, ~ ( 2 s =) 0.01, ~ p(2pJC = 0.15, and p(2pX),, = 0.14, values which are close to the spin populations estimated from values of Ai, and A,, for "AI, "C, and "0. We have performed INDO calculations on AI(CO), using the same interatomic distances as (IS) Melamud. E.; Silver, 8. L. J . Phys. Chem. 1973, 77, 1896-1900.
J. Phys. Chem. 1986, 90. 1528-1530
1527
Kasai and Jones'but varying the angle 8, the results of which are given in Table Ill. Comparison of the computed and estimated values of p(3s),, and p(2pJC suggest a value of0 somewhat greater than I I O o whereas that ofp(3px)Al is consistent with the lower value. Interestingly, our I N D O calculations give p(2p,), > p(2pJC whereas Kasai and Jones' calculation' gives similar unpaired spin populations in the 2p, orbitals of carbon and oxygen. A possible alternative structure for AI(CO), is 2, Le., a structure
AI
Ix-
ob0ob
to the 'A," representation of C, symmetry. Although we cannot completely discount this structure it seems unlikely because the unique "AI and 'IC anisotropic tensors are parallel and not perpendicular to each other as would be expected for structure 2. This conclusion is supported by consideration of the simplified molecular energy level schemes for AI(CO), with C2, and C, symmetry (Figure 5). In the bent C,, structure the 3s orbital ( a , ) interacts with the 5a(al) ligand orbitals and the 3p,(h,) orbital interacts with two 2a*(b,) ligand orbitals (the p,(al) and p,(b2) orbitals are not shown for clarity). The energy of the b, MO falls below that for the antibonding a , MO and the odd electron is placed in the former. In the sandwich C, structure the lowest-lying a , orbital interacts with two Ir(a,) ligand orbitals and the 3p, orbital interacts with two 2 r * orbitals and the odd electron in placed in the latter MO. The C , structure is clearly more favorable than the C, structure.
which is analagous to the one for CU(C,H,),'~ and which belongs
Acknowledgment. C.A.H thanks SERC for a studentship. We thank NATO for a collaborative research grant (No. 442/82) and Dr. J. S. Tse for performing the I N D O calculations. Registry No. AI(CO),, 12691-52-0 AI. 7429-90-5; CO. 630-08-0.
(16) Kasai, P. H.: McLead. D..Jr.; Watanabe.T.J. Am. Chem.Soe. 1980. IOZ. 179-190.
(17) Jorgcnscn. W.L.:Salem. L. The Organic Chemisf Bwk of Orbilals: Academic Press: New York. 1973; p 78.
2
Infrared Spectrum of the Tritiated Hydroxyl Ion (Or)in a Neutron-Irradiated LiF Crystal Yasuyuki Aratono, Mikio Nakashima, Masakatsu Saeki,* and Enzo Tachikawa Department of Chemistry. Japan Atomic Energy Research Institure. Tokai-mum. 1baraki-ken 319- / I . Japan (Receiued: August 13. 1985)
Infrared absorption of the tritiated hydroxyl ion (OT-) in a LiF crystal has been studied. The dominant absorption occurs at 2225 cm-'. Spectroscopic constants are determined on the basis of the anharmonic oscillator mcdel for a diatomic molecule. The results suggest a smaller anharmonicity of the O-H and 0-T stretching vibrations in the LiF crystal compared to those in TiO,, a-Al2OJ.and KTaO,.
Introduction In the course of the study on tritium centers in neutron-irradialed LiF crystals,' the authors attempted to observe an infrared spectrum due to the tritiated hydroxyl ion, OT-. which is the isotopic analogue of OH- and OD-. Though three reports have been published on the infrared spectra of O T , they always dealt with oxides such as Ti02? a-Al,O,.' and KTaO,? in which tritium was introduced thermally. The present paper will describe the experimental observation of the O T spectrum, being the first report on O T in alkali halide crystals, as far as the authors know. Experimental Section Single crystals of LiF were purchased from the Horiba Co. Ltd. Impurity analysis by inductively coupled plasma atomic emission spectroscopy detected 144 and 67.5 ppm of Mg and Ca. Thermal neutron irradiation was carried out in the S-pipe of a JRR-4 with a flux of 5.5 X I O " m-'.s-' for 6 h a t ambient temperature. The size of a sample for neutron irradiation was typically 5 X 10 X I mm'. The concentration of tritium in a ( I ) Y. Aratono. M. Nakashima. M. Sacki, and E. Tachikawa, Radioehim. A m . 37, 101 (1984). (2) J. B. Bates and R. A. Perkins, Phys. Reo.. 16, 3713 (1977). (3) H. Engstrom, J. R. Rala. J. C. Wang. and M. M. Abraham. Phys. Reo.. 21. I520 (1980). (4) H. Engstram. J. B. Bates, and L. A. Boatner. J. Chem. Phys., 73, 1073 (1980).
sample was (8.2 f 2.0) X IOs Bqmg-' which was determined by dissolving the irradiated sample in a nitric acid solution. (This concentration corresponds to (1.2 f 0.3) X T atoms.cm-J.) The irradiated sample was annealed in a quartz tube equipped with an electric furnace in a stream of H e for 15 min at various temperatures. Subsequently, the sample was subjected to infrared absorption measurement. Spectra were obtained with a conventional double-beam infrared spectrophotometer, Hitachi Model 260-50, in the frequency range 250 to 4000 cm-l at room temperature. The stated resolution was 1.5 cm-' at 1000 c d . +radiation was carried out with a *Co source at room temperature. The dose rate was 4.1 X IO' C.kg-'.
Results Figure I shows the infrared spectrum o f a sample annealed at 650 OC after neutron irradiation in the frequency region of OHand O T together with that of a nonirradiated sample. In the latter spectrum, two strong absorption bands at 3578 and 3614 cm-' and a weak one at 3649 cm-' were observed in the OH- region (scatrum Al. while in the O T reeion no absorotion was detected (spectrum i j . I n the samole. tritium atoms were mainlv formed by the 6Li(n,,,a)T reaciion. The resulting a and triton particles initially possess kinetic energies of 2.06 and 2.73 MeV, respectively. Thus, radiation damage in the sample was caused by these particles as well a s by y-rays and neutrons in the reactor. As a result, the irradiated sample was black due lo color centers. However, almost
~.
-
0022-3654/86/2090-1528%01.50/0 0 1986 American Chemical Society
