3539
J . Phys. Chem. 1990, 94, 3539-3543 V. Summary and Conclusions Results of these studies for 7AZI and its clusters with argon, methane, water, alcohols, and ammonia can be summarized as follows: ( I ) the spectrum of the bare molecule is quite congested with many low-energy vibronic features and apparent doublets appearing in the mass-resolved excitation spectrum: ( 2 ) the cluster spectra are also of the same nature but the vibronic structure is quite different for each particular cluster: ( 3 ) the vibronic doublets disappear upon deuteration of 7AZ1 at Hlo; (4) calculations suggest that 7AZI may be nonplanar in SI;(5) potential energy calculations suggest that cyclic hydrogen bonding between water, alcohols, and ammonia and 7AZI is not a stable low-energy structure for the first two solvent molecules in the cluster; and ( 6 ) all cluster spectra are red-shifted from the bare molecule origin, especially for polar solvents as indicated in Table V. The conclusions we draw from these results are as follows: ( I ) strong nT*-m* vibronic mixing occurs between the ‘Lb T X * excited state and a lower lying n r * state; (2) this vibronic mixing is only partially removed by clustering; (3) a cyclic hydrogenbonded structure is not achieved for the 7AZI(H20)I,2,7AZI(ROH)l,2,or 7AZI(NH3)1,2clusters; and (4) the 7AZI molecule
is nonplanar (at Hlo-NI on the pyrrole ring) in the first excited singlet state. The large spectral SI So red shifts for water, alcohol, and ammonia 7AZI clusters do not necessarily imply a cyclic hydrogen-bonded cluster structure because the (supposed) stability of these structures would not correlate with the size of the red shift. Finally, these results suggest that the double-hydrogen-bonding structures assumed to be intermediates in the 7AZI tautomerization process in condensed phases may only be present in rather high solvent density systems for which solvent molecules occupy the ring-centered solvation sites of 7AZI. Under such conditions, solvent molecules may occupy the lower binding energy hydrogen-bonded sites at the N, and Nl-Hlo positions (see Figure la).
-
Acknowledgment. This work is supported in part by grants from N S F and O N R . We thank Professor J. M . Hollas for helpful communications on 7-AZI spectroscopyand for discussion of some of his unpublished results on 7-AZI. Registry No. 7AZ1, 271-63-6; Ar, 7440-37-1; CH,, 74-82-8; H 2 0 , 7732-1 8-5; D20,7789-20-0; CH30, 67-56-1; C2H50H,64-17-5; NH,, 7664-41-7; D2, 7782-39-0; H2, 1333-74-0.
Au( 0)-Ethylene and Au( 0)-Dioxygen Complexes: Gold Nuclear Hyperfine and Quadrupole Coupling Tensors Paul H. Kasai IBM Almaden Research Center, San Jose, California 951 20 (Received: September 22, 1989)
Powder pattern electron spin resonance spectra of the Au atom complexes of ethylene and oxygen molecules, Au(C2H4)and Au(02), show anomalies due to 1 9 7 Anuclear ~ quadrupole interactions. I n the case of Au(C2H4).owing to an extremely large nuclear hyperfine interaction, the quadrupole interaction is manifested as a subtle intensity variation in the powder pattern of the normal transitions. In the case of Au(02),the hyperfine interaction is small, and the quadrupole term is manifested in the form of forbidden transitions. Both complexes have a side-on structure where the metal atom is situated equidistant from the ligand termini. The following quadrupole coupling tensors were determined through analyses of the powder patterns via simulation: for Au(C2H4)P, = -2Py = -2P, = 100 h 1 5 MHz, and for Au(02) P , = -2P, = -2PY = 45 5 MHz, where the z axis is parallel to the C-C or 0-0 bond of the ligand and they axis is perpendicular to the plane of the complex. The electron distributions indicated by the quadrupole tensors are in accord with the structures and the bonding schemes envisaged for the respective complexes.
*
Introduction Earlier we reported on ESR studies of mono- and bis(ethy1ene) complexes of Au atoms.’ The complexes were generated by cocondensation of gold atoms and ethylene molecules in argon matrices at near liquid helium temperature. The ESR spectrum of Au(0)-monoethylene is characterized by an extremely large, essentially isotropic hf (hyperfine) coupling tensor of the 1 9 7 A ~ nucleus (natural abundance = loo%, I = 3 / 2 , = 0.1439@”).The ESR spectrum of bis(ethylene)gold(O), on the other hand, is characterized by an orthorhombic g tensor of large anisotropy, and a relatively small but highly anisotropic 1 9 7 Ahyperfine ~ (hf) coupling tensor. The I9’Au nucleus has an unusually large nuclear cm2), and hence when quadrupole moment (Q = 0.594 X its hyperfine interaction is small, conspicuous anomalies may be seen in the ESR powder pattern due to severe mixing of the nuclear spin states. Such anomalies are observed in the powder pattern of bis(ethylene)gold(O) and have been analyzed in detaiL2 When the hyperfine interaction is extremely large compared to the nuclear quadrupole term, the effect of the latter upon the ESR powder pattern is almost negligible and difficult to discern. During our recent study of Au(0)-monocarbonyl, we noted and ( I ) Kasai, P. H. J . Am. Chem. SOC.1983, 10.5, 6704. (2) Kasai. P. H. J . Phys. Chem. 1988, 92, 2161.
0022-3654/90/2094-3539$02SO/O
showed that, even in such a case, the quadrupole term causes a subtle but characteristic deviation in the powder pattern of the normal transitions, and it may be assessed with reasonable accuracy by spectrum ~ i m u l a t i o n . ~ Recently we also reported on the ESR spectrum of Au(0)dioxygen ~ o m p l e x . Though ~ ethylene and oxygen molecules are nominally “isoelectronic”, and both Au(C2H4)and Au(0,) complexes have similar symmetric side-on structures, the interactive schemes and dispositions of the unpaired electron in these complexes are totally different as depicted be lo^.^,^*^-' V
(3) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1985, 107, 6385. (4) Kasai, P. H.; Jones, P. M. J . Phys. Chem. 1986, 90, 4239. ( 5 ) McIntosh, D.;Ozin, G. A. J . Orgonomet. Chem. 1976, 121, 127. (6) McIntosh, D.; Ozin, G.A. Inorg. Chem. 1976, I S , 2869. (7) Howard, J. A.; Sutcliffe, R.; Mile, B. J . Phys. Chem. 1984, 88, 4351.
0 1990 American Chemical Society
3540
The unpaired electron in Au(C,H,) is in the sp, orbital of Au; the complex is held by the dative interaction between the filled a, orbital of the ligand and the vacant sp, orbital of Au. The A u ( 0 2 ) complex is a charge-transfered, ion-pair complex. It is held by the resulting Coulombic interaction, the dative interaction between the filled ax orbital of 02-and the vacant s orbital of Au, and possible back migration from the doubly occupied a,* orbital into the p, orbital of Au. The unpaired electron is in the T,* orbital. In our earlier report on Au(02), due to a poor spectral quality, we were unable to determine the 1 9 7 Ahyperfine ~ and quadrupole coupling tensors with satisfactory a c ~ u r a c y .I ~t would be elucidative to determine both the hyperfine and quadrupole tensors of the I9’Au nuclei in these complexes accurately. They should reveal, respectively, the unpaired electron distributions and the electric field gradients at the Au nuclei consistent with the bonding schemes discussed above. The present paper describes the assessment of the quadrupole tensor of Au(C2H4)from the E S R powder pattern reported earlier,] and the same of Au(02) from a spectrum of better quality obtained thenceforth.
Experimental Section A liquid helium cryostat that would allow trapping of vaporized metal atoms in an inert gas matrix and examination of the resulting matrix by ESR had already been described.8 For the present series of experiments gold atoms were generated from a resistively heated Ta cell (1500 “C) and were trapped in argon matrices containing controlled amounts of oxygen. In this type of experiment, compared to other group 1 1 metal atoms (Cu and Ag), gold atoms consistently required a high ligand concentration (- I O mol 72) for the formation of complexes. However, owing to the paramagnetism of oxygen molecules, the ESR spectrum observed from the Au/oxygen( 1 O%)/Ar system was too broad to discern the relevant structures. An argon matrix showing the ESR powder pattern of A u ( 0 2 ) with reasonable intensity and resolution was presently obtained by using argon mixed with 7 mol % oxygen. The ESR spectrometer used was an IBM Model ER200D. A low-frequency modulation (375 Hz) was used for the signal detection. The spectra presented here were observed while the matrix was maintained at -4 K. The spectrometer frequency locked to the cavity was typically 9.428 GHz, and the microwave power level was -20 pW. Theoretical Analysis The structures of Au(C2H4)and Au(0,) depicted above are such that the directions of the principal axes of the g tensor and the 1 9 7 Anuclear ~ hyperfine and quadrupole tensors, A and P, are all coincident. The spin Hamiltonian which would adequately describe the ESR transitions of these complexes is then given as foll0Ws.g @spin
Kasai
The Journal of Physical Chemistry, Vol. 94, No. 9, I990
= P(gxH.3, + g,H.Sy + g,H,S,) + Ax[.$, + AyI2-y + A,I,S, 4- P,I.: + PJ? PJZ2 (la)
+
Equation 1 b follows from eq la due to the traceless nature of the quadrupole tensor ( P , + Py + P , = 0). As stated earlier, for Au(C2H4).the 19’Au hyperfine coupling tensor is extremely large (-650 G), but essentially isotropic; the powder patterns of individual hyperfine components are thus determined mostly by the g tensor anisotropy. The forbidden transitions ( A m , = f l , f 2 ) are weak, and the resonance positions of the normal transitions are given by the second-order solutions of eq 1 with reasonable accuracy. Let us assume, for the sake of brevity, that the g, A. and P tensors are all axially symmetric. (8) Kasai, P. H. Arc. Chem. Res. 1971, 4, 329. (9) See, for example: Abragam. A.; Bleaney, 9 . EIectron Paramagneric Resonance of Transition Ions; Oxford University Press: London, 1970; pp 178-186.
The resonance position of the normal hyperfine component H ( m ) where mi = m is then given as follows. H ( m ) = HO- A m - 7 1 - 772 - h
+ (2
(2)
Here 7’s and l ’ s represent the second-order contributions of the hyperfine and quadrupole tensors, A and P, respectively.
t2 =
-[
8pl2gIl2 AllA,glI2g, Ag2
A2g4
*]
2
cos2 8 sin2 0 [ [ ( I
+ 1 ) - 2m2 v 4 1 m (4b)
Here A‘s and P s are in gauss and Ho = hv/gP PI1 = 72pz
g2 = gl12 cos2 8
+ gL2sin2 8
where 8 is the angle between the symmetry axis and the magnetic field. Equations 2 and 4 reveal immediately the well-known result that the resonance positions of the parallel components are totally unaffected by the quadrupole term. These equations also show that, in the present case of I = 3 / 2 , the perpendicular positions of the lower field hyperfine components (mi = 3 / 2 , and 1 / 2 ) are ~ A )those of the high-field shifted downfield by w - ~ P , , ~ / (and components (m, = -3/2, and - l / 2 ) are shifted upfield by the same amount. This shift due to lI is very difficult to discern and determine from the powder pattern when A is extremely large (hence large second order terms q’s), and the g tensor and/or the A tensor have anisotropies similar to or larger than this shift. The nuclear quadrupole effect given by l2vanishes when the magnetic field is along a principal axis (8 = 0 or a / 2 ) . For I = 3/2, the quantum state dependent factor [ [ ( I + 1) - 2m2 - 1/4]m in eq 4b is equal to + 3 / 2 if m = -3/2 or + 1 / 2 and is equal to - 3 / 2 if m = + 3 / 2 or -1/2. Hence, if gll > g,, the quadrupole term E2 would shift the signals of intervening orientations toward the gl, positions for the hyperfine components H ( + 3 / 2 ) and H(-1/2) and away from the gIlpositions for the hyperfine components H ( - 3 / 2 ) and H ( + 1 / 2 ) . These shifts should affect the relative intensities of the parallel and perpendicular signals of the powder pattern in a characteristic manner. These conclusions stand even if the g and A tensors are orthorhombic so long as the quadrupole tensor remains axially symmetric. As is shown later, such deviation of the powder pattern is discernible and can be utilized to assess the quadrupole coupling tensor with reasonable accuracy. As stated earlier, in the case of Au(02), the hyperfine coupling tensor is small ( - 3 0 G) and the second-order solutions of eq 1 do not adequately describe the effect of the quadrupole term. Owing to a large anisotropy of the g tensor, the overall powder pattern of Au(0,) is determined by the g tensor, and the signals corresponding to each principal g axes are split by the hyperfine and quadrupole interactions manifested along the respective directions. When H = Hi, and Hj = H k = 0 (where i , j , k = x, y , z ) , an exact analytical solution of Hamiltonian ( I ) is possible. The result
The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 3541
Au(0)-Ethylene and Au(0)-Dioxygen Complexes a / o / 6 / -3
/
0
-3
12 / -6 / -6
18 / -9 / -9
211 1-12 /-I2
30 1-15 1-15
I / I / I 1
I I I / l /
i / I / l 1
I
0
1
/
I I 1 1
I
/ l I l / /
I 1
I
,
I/ 1
1
I
II
/I
I
I
I I i,
Ill
/ I /
L
I
IZI
J
I
I
1
PZ / PX / PY
1
I 2
for the case I = 3 / 2 yields the resonance fields of the normal hf components and the forbidden transitions of the type Ami = f 2
“ 2
I 3
I
I
I
4
I 5
KG
[XI RND IT1
Figure 1. Hyperfine multiplets computed for cases when the magnetic field is parallel to each of the principal axes of a model system with g, = g, = gv = 2.00, A, = A, = A, = 30.0 G , and axially symmetric quadrupole coupling tensors given at left (in gauss).
I
.’..”
Figure 2. ESR spectrum observed from Au/C2H4 (10%)/Ar matrix. The signals A, B, and C are due to isolated Au atoms, Au-monoethylene, and Au-diethylene complexes, respectively.
transition probabilities of the normal and forbidden transitions computed for a model system when the magnetic field is parallel to the respective principal axes. The model assumed an isotropic g tensor (g, = g, = g, = 2.00), an isotropic hyperfine coupling tensor of A, = A, = A, = 30.0 G, and an axially symmetric quadrupole tensor, PzlPxlPy,shown in the left column. The uppermost set of patterns in Figure 1 is for the trivial case of no quadrupole interaction; the spacings within each quartet are hence Axy,rof 30.0 G. The following five sets show the effect of increasingly larger, axially symmetric quadrupole terms. The z axis is the symmetry axis. As expected, the splittings along the symmetry axis remain unperturbed. In the perpendicular directions the normal hf components “move out”, thus making the inner spacing larger, and lose their intensities, while the forbidden transitions (of Ami = f 2 ) emerge in the central region. It should be noted that, when the magnitude of the quadrupole term is 1 / 3 of the hyperfine term, the perpendicular signals may appear like a quintet of the binomial intensity ratio. When the magnitude of the quadrupole term is comparable with that of the hyperfine term, the perpendicular signals are dominated by the forbidden transitions and appear as an 1 : 2 : l triplet.
-
( l j 2 ) [ ( A i - 3PJ2 pi
= (1 / 2 ) [ ( A i
+ 3Pi)’ + 3(Pj - P k ) 2 ] ’ / 2( 1 / 2 ) [ ( A i - 3Pi)2
+ 3(Pj - Pk)2]1’2
+ 3(P, - P/y]I/’
The forbidden transitions of the type Am, = f l do not occur when the field is along a principal axis. The above solution neglects the second-order effect of the hf term and the nuclear Zeeman term. Their effects are negligible in the present case. Examination of eq 5 for the normal hf components reveals that the outer spacing remains exactly at A i and the central spacing is given by 2K; - Ai. It can be readily shown that K~ 2 A i (the equality holds only when Pi = Pk). Thus when the magnetic field is along a principal axis, the outer spacing is always smaller than the inner spacing (except for the special case of P, = Pkwhereby all the spacings are equal to Ai). The eigenfunctions obtained from Hamiltonian ( I ) for M , = + I / 2 and M, = -1 / 2 would have the general forms of @+(mi) = x:,+!-,C,,,kJk) and @-(m,,) = Ck’f-,C,,,,lk), respectively. Here Ik) represents the nuclear spin state of m, = k . The probability of the ESR transition H(m+m’) is then given by [ ~ ~ ~ - , C m k C , , Figure , f k ] 2 .1 shows the resonance positions and the
Spectral Analyses Figure 2 shows the ESR spectrum observed from an argon matrix containing Au and ethylene (10 mol % ) I . Three sets of signals, A, B, and C, can be recognized as indicated. The signals A are due to isolated Au atoms. The signals B and C are due to gold(0)-monoethylene and gold(0)-bis(ethylene) complexes, respectively. The signals A had been analyzed earlier,I0 and the signals C of bis(ethylene)gold(O) were recently analyzed in detail addressing particularly to the effect of the Ig7Aunuclear quadrupole term.’ The quartet feature of the B signals is due to a large but essentially isotropic hf interaction with the 1 9 7 Anucleus. ~ The powder patterns of the individual hf components of Au(C2H4)we,: thus recognized and assigned as indicated at the base of the figure. As stated earlier, when the hf term is as large as in the present case, the resonance positions of the normal hf components are affected little by the quadrupole term. Thus from the x, y , and z positions of the lowest and highest filed hf components the following g and A tensors of the complex were determined by the usual iterative method.” g, = 1.978
A, = 1.772 GHz
gy = 1.782
A, = 1.726 GHz
g, = 1.946
A, = 1.725 GHz
The rationale for identification of the principal tensor axes with the molecular axes (as given above) was discussed in the earlier rep0rt.I (10) Kasai, P. H.; McLeod, D.,Jr. J . Chem. Phys. 1971, 55, 1566. ( 1 I ) Kasai, P. H.; McLeod, D., Jr.; Watanabe, T. J . Am. Chem. Soc. 1980, 102, 179.
3542
The Journal of Physical Chemistry, Vol. 94, No. 9. I990
Kasai
-
41
gl
1 3100 G
33OOG
W
G
Figure 4. ESR spectrum of AuOl complex observed from A@,(7%)/Ar matrix. The sharp signal at 3180 G is due to isolated Au atoms. The parallel and perpendicular signals are recognized as indicated. TABLE I: g Tensors and IWAuNuclear Hyperfine and Quadrupole Coupling Tensors of AuC,H, and AuO, Complexes
AuC2H4 Figure 3. ESR powder patterns of Au(C,H,) simulated based on the g and Au hyperfine coupling tensors given in the text and increasingly larger quadrupole tensors symmetric about the x axis. The assumed P, value (in MHz) is given at left.
It can be readily shown that, even when the hf term is as large as in the present case, the resonance positions of the normal hf components are given with reasonable accuracy (within I O G) by the second-order solution of eq 1 with or without the quadrupole term. A computer program that would simulate the ESR powder pattern based on the second-order solution of eq 1 has been described earlier.” The top trace in Figure 3 shows the spectrum simulated by this program based on the g and A tensors given above and no quadrupole interaction.I2 It is in reasonable agreement with the observed pattern in terms of the overall relative intensities of the hyperfine components and the x, y , and z resonance positions of individual hyperfine components. It is at variance with the relative intensities of the x and z signals within each hyperfine components, however. In the observed pattern (Figure 2), the x signal is the strongest in the lowest field and the second highest field hf components, while the z signal is the strongest in the second lowest field and the highest field hf components. As discussed in the theoretical section, as g, > g,, this variance can be readily accounted for by a quadrupole tensor of axial symmetry about the x axis. The remaining simulated spectra in Figure 3 demonstrate the effect of increasingly larger quadrupole tensors symmetric about the x axis. The assumed P, value is given at left of each pattern. From these simulations the I9’Au nuclear quadrupole tensor of Au(0)-monoethylene was determined as follows. P, = -2PY = -2P, = 100 f 15 MHz The ESR spectrum of AuO, observed from the A u / 0 2 (7%)/argon system is shown in Figure 4. The sharp isotropic signal at 3 180 G is the second highest field hyperfine component of isolated Au atoms. As concluded in earlier reports, the ESR spectrum of AuO, is essentially that of an superoxide ion, 02-, the degeneracies of the K and K* orbitals of which are lifted by the Au’ ~ a t i o n . The ~ . ~ g tensor of the complex is thus given by (12) In simulating the spectra a Lorentzian line shape was assumed, the line width of which was given by W ( m ) = W, SA(m ( A / H a ) [ I ( I I ) m2]1. Here Wa represents the inherent line width, and SA represents the
+
+
+
inhomogeneity of the large isotropic coupling constant in the matrix environment. For patterns in Figure 3 it was set W , = 8 G. and SA = 5 G.
Y
V
z
g tensor
1.978 (2)
A , MHz
1.946 (2) 1713 (6)
A , MHz
1773 (2) lOO(15) 1.984 (3) 92 (3)
1.782 (2) 1716 (6) -P,/2 1.984 (3)
P, MHz
-PJ2
P, MHz AuO,
g, = 2.00
g tensor
92 (3)
-PzP
-Px/2 2.104 (3) 94 (3) 45 (5)
+
2X/6E, and g, = g,, = 2.00. Here X is the spin-orbit coupling constant of oxygen atom, 6E is the separation between the K,* and K,,* orbitals, and the z axis parallels the internuclear direction of the dioxygen moiety as depicted in the Introduction. The gll (=g,) signal and the g, (=g, = g,,) signal were hence recognized as indicated in the figure, wherein both signals were ~ hyperfine and quadrupole insplit further by the 1 9 7 Anuclear teractions manifested along the respective directions. We note immediately that the g, signal has a quintet pattern. The presence of a quadrupole term, which is symmetric about the z axis and the magnitude of which is 1/ 3 of the hyperfine term, is thus strongly indicated. The ESR spectrum of AuO, (Figure 4) was analyzed accordingly, the g tensor given by the central positions of the gl,and g, signals, and the hyperfine tensor determined from the total spread of the parallel signal and the outer spacings of the perpendicular quintet. g, = 2.104 (3) g, = g, = 1.984 (3)
-
A, = 94 f 3 MHz
A, = A, = 92 f 3 MHz
-
When the magnitude of the quadrupole term is greater than 1 / I O of the hyperfine term, spectrum simulation based on the second-order solutions of eq 1 is no longer possible. A powder pattern simulation program based on complete diagonalization of eq 1 has been described in earlier report^.'^.'^ Shown in Figure 5 are the spectra simulated based on the g tensor and the I9’Au hf coupling tensor given above and increasingly larger, axially symmetric quadrupole tensors given at left of each trace. The quadrupole tensor of AuO, was thus concluded to be P, = -2P, = -2PY = 45 f 5 M H z Discussion The g tensors and the 1 9 7 Anuclear ~ hyperfine and quadrupole
coupling tensors of AuC2H4and AuO, complexes presently determined are summarized in Table l. The g tensor and the hf ( 1 3 ) Kasai, P. H.: Jones, P. M. J . Am. Chem. SOC.1985, 107, 813. ( I 4) Lund, A,; Thuomas, K.: Maruani, J J . Magn. Reson. 1978, 30,505.
J . Phys. Chem. 1990, 94, 3543-3541
3543
-2eqyy = ( 4 / 5 ) e ( F 3 ) , and , if it is the only electron in the valence shell, the quadrupole coupling tensor in the form of eq 1 would be as follows.
0 MHz
/
Substituting the known quadrupole moment of the 1 9 7 Anucleus ~ cm2) for Q and 5.19 au-3 for (r-3),,u,6p(the value (0.59 X estimated from the fine structure interval of Au atomsI5) one obtains P: = 96 MHz. Neglecting the shielding effect of the core electrons, the observed quadrupole tensors then indicate the electron population of 1 .O in the Au p, orbital of AuC, H4 and -0.5 in the Au pz orbital in the case of AuO,. Analysis of the hyperfine coupling tensor of AuC2H4reported earlier showed the unpaired electron density of -0.5 in the Au px orbita1.l This is significantly less than that given from the analysis of the quadrupole tensor. The difference may be construed as an evidence for the dative interaction between the bonding 7,orbital and the vacant sp, orbital of the Au atom. In A u 0 2 the unpaired electron is in the antibonding rY*orbital and does not contribute to the electric field gradient a t the Au nucleus. The dative interaction of the electrons in the bonding A, orbital is more likely to involve the Au 6s orbital only as the latter is totally vacant in the A u + 0 situation. ~ Back migration that would generate electric field gradient of consequence at the Au nucleus is that from the doubly occupied r X *orbital of oxygen into the Au pz orbital. The electron population of -0.5 in the Au pz orbital concluded in the anlysis of the quadrupole tensor indicates the extent of such migration.
-
50 MHz
7 7 Figure 5. ESR powder patterns of AuOl simulated based on the g and Au hyperfine coupling tensors given in the text and increasingly larger
quadrupole tensors symmetric about the z axis. The assumed P, value (in MHz) is given at left. tensor of AuC2H4 were redetermined corrected for the shift of ) 6.5 G . the perpendicular signals by 3 P I l 2 / ( 2 AN The electric field gradient eq at the nucleus due to an electron in a valence p z orbital, for example, is given by eqrz = -2eq,, =
Registry No. Au(C2H4),61943-23-5; Au(02), 60294-90-8.
(15) Moore, C. E. Natl. Stand. Ref Data Ser. 1971, 35.
(US.Natl. Bur. Stand.)
Gas-Phase Inorganic Chemistry: Laser Spectroscopy of Calcium and Strontium Monocarboxylates L. C. O’Brien,’ C. R. Brazier,$ S. Kinsey-Nielsen, and P. F. Bernath*.s Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: October IO, 1989)
The calcium and strontium monocarboxylate_frze iad_icalsyere_made by the gas-phase reaction of the metal vapors with carboxylic acids. Three electronic transitions, A-X, B-X,and C-X,were detected by laser-induced fluorescence. Metal-ligand stretching frequencies were derived from these low-resolution spectra. The carboxylate ligand binds in a bidentate manner to the metal.
Introduction pyrrolate,13 and m ~ n o f o r m a m i d a t ewere l ~ all recorded in this laboratory. These new molecules were Produced by the W-Phase As part of a continuing study of gas-phase alkaline-earthcontaining free radicals, we report here on the metal monocarboxylate derivatives. The laser-induced fluorescence spectra ( I ) Brazier, C. R.; Ellingboe, L. C.; Kinsey-Nielsen, s.; Bernath, P. F. J . of calcium and strontium monoalkoxides,’ monoalk~lamides,~-~ Am. Chem. SOC.1986. 108, 2126. OBrien, L. C.; Brazier, C. R.; Bernath, P. F. J . Mol. Spectrosc. 1988, 130, 33. monoalkylthi0lates,4~~ monomethyl,6 m o n o a ~ e t y l i d e monoiso,~~~ (2) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. J. Phys. Chem. monocyclopentadienide,12mono~ y a n a t e , ~monoazide,” J~ 1987, 91, 2179. ‘Current address: Food and Drug Administration, Division of Drug Analysis, 1 1 14 Market Street, St. Louis, MO 63101. ‘Current address: Astronautics Laboratory/LSX, Edwards Air Force Base, CA 93523. Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar. *To whom correspondence should be addressed.
0022-3654/90/2094-3543$02.50/0
(3) Brazier, C. R.; Bernath, P. F. Work in progress. (4) Fernando, W. T. M. L.; Ram, R. S.; Bernath, P. F. Work in progress. (5) O’Brien, L. C.; Ram, R. S.;Bernath, P. F. Work in progress. (6) Brazier, C. R.; Bernath, P. F. J . Chem. Phys. 1987,86,5918; J . Chrm. Phys. 1989, 91.4548. (7) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chcm. Phys. Lert. 1987, 136, 97.
0 1990 American Chemical Society
