HCr, LiCr, and NaCr molecules: ESR and ground state properties

Volume 113. number

CHEMICAL PHYSICS LETTERS

6

HCr, LiCr, and NaCr MOLECULES: RJ

VAN ZEE,

Owmicn! Physics Rcccivcd

5

CA.

BAT-MANN

ESR AND

GROUND

’ and W_ WELTNER

Center and Departnmmf

of C7temiby.

STATE

8 February 1985

PROPERTIES

Jr.

University of F7orido. Gainesville. Florida 32611.

USA

November 1984

ESR spectra of %r’H,

‘Lis3Cr,

and 23 Na’kr

molecules isolated in argon matrices at 4 K have been dnalyzed to yield g

tensor. hyperfmc. and zero-field splitting parameters.lbe latter are found to be IDI = 0.34.0 065, and 0.047 cm-‘. respecbvcly Bonding and ionicrty in the three % molecules ye discussedrelativeto the observed parameters.

l_ Introduction

2. Experimental

In the course of studres of transition-metal molecules [l] and their hydrides [2,3], it was of interest to compare lithides with hydrides. A first member of that series, ScLi, has recently been considered theoretically by Harrison [4] who has also ConJectured about the bondmg in other transrtron-metal lithrdes A natural extensron of this was to also observe the trend in electronic properties among the aUcali-metal/transitionmetal dratomics. This paper represents a modest probe LII that direction_ Attempts were made to observe other diatomics containing SC or Mn rather than Cr, but the ESR spectra of I.&I and NaM were not detected in those cases, either because the molecules did not form or the spectra were intrinsically weak. CrH has been previously investigated in this laboratory viz ESR [3] and found to be a 6x molecule with

Separate resrstance-heated tantalum cells were employed for the simultaneous vaporization of the two metals [5]. Chromium was vaporized at 1275”C, lithium at =600°C, and sodium at =350°C_ The chromium vaponzation temperature was measured with an optical pyrometer, uncorrected for emissivity. These metals were vaporized into matrices of argon (Airco. 99_999% pure) deposited onto a flat white sapphire rod majntamed at 4-6 K, but capable of bemg raised to higher temperatures_ The furnace has been described in detail previously [6] _ The cryostat is a HeliTran unit with a movable assembly modeled after that of Knight et al. [7] _ The ESR equipment is a new IBM/Bruker 200D SRC series mstrument operating in X-band *_ Natural chromium [99.999% pure: 9.55% 53Cr(I = 3/2), 90.45% other isotopes (I = 0)] was pur-

a zero-field splitting (zfs) parameter ID I = 0.34( 1) cm-l _Hyperfrne structure (hfs) was discernible but unresolved so that only estimates of the interaction constants could be made. Reported here are more accurate 53C,(l = 3/2) hf s measurements by preparation of the molecule enriched wrth 53Cr_ The results of this research mdicate that LiCr and NaCr are closely similar in their electronic properties and differ significantly from the CrH molecule.

chased in flake form from Spex. 96.98% rsotopically pure 53Cr metal powder was obtained from the Stable Isotopes Drvrsion at Oak Ridge National Laboratory, Oak Ridge, Tennessee. Lithium [999% pure: 92.6% ‘Li(I = 3/2), 7.4% 6Li(I = 1) was purchased from Fisher, and sodium [99.9% pure: 100% 23Na(I = 3/2)J from J.T. Baker.

’ Resent address: Department of Cbenust~y. University of Scranton. Scranton. Pennsylvania18510, USA

* To be described in more detail in a forthcoming on the ESR of chromium specks in matrices_

524

0 009-2614/85/S (North-Holland

publication

0330 0 Elsevier Science Publishers B-V_ Physics Publishing Division)

Volume

113. number

6

CHEMICAL

PHYSIC5

3. ESR spectra

LETTERS

8 February

largely unaffected and is considered tive of the isolated CrH molecule.

1985

to be representa-

X2. LiCr andiVaCr Fig. 1 shows clearly the %r(I = 3/2) hyperfme splitting of 18( 1) G of the very strong perpendicular (xyl) line of the CrH molecule prepared with the en-

Figs. 2 and 3 show the similar observed

riched isotope. IIus is in close agreement with the earlier value of 19(2) G estnnated from the barely discernible hf structure on the unenriched line [3]. The lH hfs lies within the s3Cr linewidths and therefore

spectra

of

LCr II

II

III

I

II

II

I rl_z

It must be Judged to be less than =$I G, much smaller than the earlier estimate of 16(22) G. The other three fine-structure lines at higher fields are much weaker than this line and the detection of hfs was not definite. (We also note here the error in our previous study [3] 111assigning the persistent line at 1670 G in these matrices to a possible quartet CrH3 molecule It is a “halffield” line [8] of Cr atoms whxh appears whenever the atom signal 2t g = 2.0 is strong.) The clearly resolved line in fig. 1 is obtained in a matrix contammg a relatively high concentration of Cr and at a power of 13 mW. In general this line exhibited cverlapping patterns of the hfs m fig. 1, and its apparance and position was dependent upon the orientation of the rod m the magnetic field, the microwave power, and the chromium concentration. Thus, there are usually several sites for CrH in the matrix and a range of interactions with both argon and chromium species; however, the hf spacing in fig. 1 appears to be

I

30

20

,

I

40

50

H(KG) Fig. 2. ESR spectrum of the LiCk matrix at 4 K. Lines attributed to at the top of the spectrum. L.i and around 3.4 kG. are also indIated

I

25

Fig. 1. ESR spectrum of the strong perpendicular (xyl) line at f = 6 of rhe 53CI’H molecule isolated in an argon matrix at 4 K, measured at relatively high power (v = 95556 GHz).

1

IO

molecule isolated in an argon the molecule are Indicated 0 atom s~~nsls, entered (Y = 9.3750 GHz).

N&r

I

I

30

40

45

Fig. 3. ESR spectrum of rhe “Na5%r molecule isolated in an argon matrix at 4 K. Lines atulbuted to the molecule are indi-ted at the top of the spectrum. Strong %Y atom and CH3impurity signals occur at about 3.5 kG. Two sets of sodium atom signals are designated Na and Na’ (see text). Y = 95546 GHz

525

Volume

113. number 6

CHEMICAL

PHYSICS

these molecules trapped m argon matrices at 4 K_ In both cases the lines are broad (SO and 65 G, respectively, for LlCr and NaCr), and no alkali-metal hfs was observed_ When enriched with 53Cr, the strong line at 2580 G in the LiCr spectrum exhibited weak but definite hfs from which one finds IA(53Cr)I =6-S(3) G A search for similar hfs structure on the Nas3Cr lines was not successful_ In the Na + Cr spectrum there is the appearance of two distinct sets of 23Na (1= 3/2) atom hyperfine lines, designated as Na and Na’ in fig. 3. The Na quartet has the well-known spacing and site structure for sddium atoms isolated in an argon matnx [9,10] - The Na’ signals have not been observed before but are completely reproducible, although not appearing with the same relative inter;sity in all argon matrices.

4. Analysis For all three

of these %Z molecules

the spectra

were

+ &P(H,S,

+ H&J

+ bS(S:

8 February

1985

C 2. LiCr and NaCr For these molecules the perpendicular fine-strutture lines were fit initially without a bi parameter, but it was found that simukkeous fitting of parallel and perpendicular lines required inclusion of small bt parameters. For LiCr, the same general pattern of lines was observed as for NaCr, but the spectrum contamed almost twice the number of resolved lines with about onehalf the width It was found that all of these could be accounted for (see table 1) by a large number of “extra knes”. This also suggests that the width of some of the NaCr lines might be due to overlapping offprincipalaxis transltlons. All of the observed Lines of NaCr could be assigned and readdy fit, m&ding one off-principal-axis line, as shown in table 2_ As noted above there was the interesting appearance of a second quartet of sodium atom lines in the matnx, designated Na’ in fig. 3_ Those designated as Na have the usual spacing of 23Na(l = 312) atoms

iit using the spin Hamlltonian: H=gnPH,S,

LEl-TJZRS

isolated

in an argon matrix

with A z 300

G 191,

- E) Table 1 Observed and calculated ESR line posltlons (in G) and derived magnetic parameters for the LiIlr molecule isolated in solid argon at 4 K tv = 9 3750 GHz)

where bq = D and bi is a higherarder tine-structure parameter [ 1 1 ] _The small hf spli ttmgs could be

8 (deg) a)

Observed

Qlculated

90

2188rS) 2580 3099 3788(20) 4821

2181 2579 3099 3782 4824

0

555(5) 1964 4748

555 1964 4748

38(l) 73 74 32 74 57

3816(S) 3886 665 1234 1265 2042

3822 3890 665 1239 1266 2037

treated by secondarder perturbation, and the 6 X 6 fine-structure matrix diagonallzed by computer, as done previously for CrH [3].

The spectra obtained here are m essential agreement wth those measured in our earlier ESR study [3], except that 53Cr hf structure was observed, yieldmg wI(%~) I = 50(3) MHz The value of the zero-field splitting parameter D = b? is determined by the positions of the fine-structure lines which may be in doubt by +20 G in some cases because of their breadth. However, an analysis of the %rH spectra corroborates the value of IDI = O-34(1 cm S-1 found in the earlier work. Efforts to also find b,b were frustrated by the uncertainties in the hi -field hne positions, but the indicaP I < 0.005 cm-‘. tions are that I b4

526

gl = 1.9990(10) gB= b, = l-gg57C5) D = 10 0651(2) cm-’ a: = *o 00011(1) IA(‘Li)l G 3 MHz IA1@2r)l = 19 MHz

cm-’

a) 8 is the angle between the molecular axis and the magnetic field.

Volume 113. number 6

CHEMICAL PHYSICS LETTERS

Table 2 Observed and =lculated ESR lmc positions (m G) and derived magneticparameters for the NaCr molecule isolated in solid argon at 4 K (Y = 9.5546 GHz) B (deg)a)

Observed

Calculated

90

2515(10) 2851 3277 379.5 4458(20)

2514 2849 3270 3804 4476

0

1364 2410(20) 4415(20) -

1374 2415 3412 4410 5451

3661

3667

40

gb = 2.0005(9) g1= 1.9985(9) b:! = D = +0.0473(S) cm-’ bs = i0.00029(9)

cm-’

a) B is the angle between the molecular axis and the magnetic Iield.

but the Na’ series must be attributed to sodium atoms perturbed such that the hfs is reduced to only ==240 G, also with an appearance of site structure_

8 February 1985

found from gas-phase spectra, within the rather large uncertamty there; 1-e. Kleman and Uhler [12] give h” = 0.03 & 0.01 cm-1 and O’Connor [13] gives I$ = 0.18 2 0.06 cm-l. Our value ofD can be expected :e he shifted from the gas-phase value by no more than about O-01. IX-II-~in the matrm environment [ 15]_ Our reanalysis of the CrH spectrum yields gl = l-935(10), lower than the earlier value of 2.005(5) [3] but still mdicating only a small difference from ge_ Thus, we would estimate AgL =gl - ge = -0.007(10) so that the spin rotation constant, Yl, as obtained from Curl’s relationship [lS] , Yb’ = -=ag,, is Yi = 0.09(10), where B = 6.1 cm-l [13] _Although not determined very accurately, this value is significant relative to the gas-phase data. (F%st application of the Curl equation to derive Y values of hydrides from ESR

data has been very successful, yieldtiig agreement with gas phase values to within *lo% [10,17] .) At the present time, the analyses of the CrH gas-phase spectra [ 12,131 have not allowed an unequivocal value of yi to be obtained, only the difference y’ - y” IS determined accurately. There is room for error in fitting the sums of combination differences as used by both sets of authors [ 181, but our data do support a small value

5. Discussion

of Yi compared to the much larger value of -yb in the excited state, in agreement tith O’Connor [13]. 5-a 7Lis3i3 and 23lVa%3

The specrra of 53Creruiched CrH have supplied a definite value of IA(53Cr) I = 50 MHz, in essential agreement with the previous estimates [3] and set the upper limit of the IH hfs at 22 MHz. ID I = I b! I derived from these spectra IS in agreement with the earlier value, 0.34(l) cm-t. The optical spectrum of CrH in the gas phase has been observed and analyzed by Kleman and Uhler [12] and by O’Connor [13] - (It has also been of interest more recently in astronomical sources [14] _) D here is equivalent to 3~; in Kleman and Uhler’s notation so that in gas-phase symbolism we find: .$ = 0.1 l(1)

cm-l

01 A”

=

E”/B

=

0.018(2).

These values are in substantial agreement with those

These molecules, not surprisingly, have similar spectra and properties. The low ionization potentials of the alkali metals suggests ionic character contributmns of the type Li%r-, where all of the unpaired spins tend to be localized on the Cr- ion_ This and s-s bondmg favor small s unpaired spins on the alkali metal, in accord with the unobservably small hfs produced

at those nuclei. The small ID I values are then also not

surprising since a spherical Cr- ion would have a zfs of Zero. One could even explam the shghtly larger value in LiCr versus NaCr as due to a slightly stronger

polari-

zation of the unpaired spin distribution by the smaller Li+ion

The increased lmewidths in NaCr spectra relative to those of IXr would ordmarily be attributed to increased matrix perturbations, but it is also possible that the groups of closely spaced lines in the LiCr spec527

Volume

113. number 6

CHE&iKAL

PHYSICS LEmER.5

tra (see fig 2) may be overlapping and unresolved in NaCr_ It is presumably this broadening that prevents the detection of %r hfs in the NaCr lines. . The second quartet of sodium atom lines in these matrix spectra, designated as Na’ in fig. 3, is interestmg. The spacmg oFfie Na signals is the usual one of about 300 G (overall splitting -1010 G) [9] whereas that of C (overall splitting -790 G). The Na’ atoms is 2240 lines for each type of sodium are spht by site structure_ The Na signals in argon have been observed by many authors and are attributed to isolated sodium atoms trapped in several sites of shghtly different ener-

8 February 1985

ZFS

Cr

of

Dlotomlcs

/

P

OF

gies [IO] _ It seems likely that the Na’ signals arise from smgle sodium atoms with chromium molecules as neighbors in the matrix. Cr, is now known to be a strongly bonded 1E molecu!e [19] which can be expected to be readily formed durmg the quenching of the matrix, and is, of course, undetectable via ESR. Other studies m tlus laboratory of argon matrices con taining vaporized chromium metal indicate that at least one other larger Cr, molecule is also present (see footnote *)_ Then the Na’ signals are attributed to a Na-Cr, species bonded only by van der Waals interactions_ The 3s electron which produces the large hfs in the isolated sodium atom is then polarized by the neighbormg Cr, molecule to a partial sp hybrid, thus reducing the s character and hfs to about SOS,. To support this picture, there is evidence of an increase in the Na’/Na signal ratio with increased Cr atom concentration in the matrix, and also after annealing.

-I’

01

02

04

03

OS

0’6

IDI (UT-’ I fig. 4. A plot of the electronegativity difference as a measure of lonicity ~crsus the measured values of the zero-lield splitting p3wmeters D (assumed positive) for four ‘E diatomia containmg a Cr atom.

As Harrison has pointed out [4] and as previously emphasized by Brewer and Winn [20], promotional energy d”s2 + d”+l s is not required m bonding Cr(d5s,

this laboratory [22], is also included_ LiCr, NaCr, and HCr all have relatively low ionicities, but the first two molecules would be expected ro have opposite dipolar character (LPCr-) from that of CrH (Cr+H-). The low value of ID 1 in L.iCr and NaCr is identified essentially with the zfs of a mildly perturbed Cr- ion, where one remembers that Cr- is isoelectronic with the %5 Mn atom for w&h D is exactly zero. At the other limit, IDI for P would be attributed to the zfs of a ds Cr+ ion severely perturbed in the crystal field of the neighboring F- ion ** The more covalent CrH molecule then has a IDI value between these two extremes.

‘S) to atoms such as Li and H. One then expects a strong s-s bond in these cases and therefore little s character in the remaining shell of five unpaired d electrons on Cr. Thus, hyperfine splittings would be expected to be small or even unobservable, as found here. Another approach to understanding the bonding in these moIecules is to consider their ionicity. If Gordy’s

The 53Cr hyperfine constants appear to exhibit a parallel variation with lonicity among these molecules Indeed IA(53Cr)l in IXr is only 19 MHz whereas it is 50 MHz m CrH. Again one notes that the dss2 s3Crion can be expected to have only the small hfs due to spin polarization. When corrected for the dfference in nuclear magneric moments of 53Cr and 55Mn, one cal-

5.3.

Bonding and romcity

mdicator

of loniclty

[21],

i.e. the difference

in elec-

tronegativlty between the two atoms, is used to correlate t-he variation among their zfs parameters, one obtains the plot shown in fig_ 4, where the value of ID I for the strongly ionic Cr+Fmolecule, obtained in 528

culates

from

approximate

the known

Mn atom

A value for s3Cr-

value (78 MHz) an

of 18 MHz, in good

** See, for example, evidence of such perturbation m the diatomic alkaline earth fluorides, ref. [24]

Volume

113. number 6

CHEMICAL

PHYSICS

agreement with 19 MHz observed for LCr. Then a value greater than about 70 MHz would be predicted for IA(53Cr) I in CrF from those of HCr and LiCr. This is close to the value of 67 MHz attributed by Kasai 1241 to an isolated Cr+ ion in solid argon. Our value of 36 MHz obtained earlier from ESR of mtCrF is probably mcorrect [22]. which is excusable because of the difficulty in detecting the signals from the 10% 53Cr present One must assume that the large value of the Cr+ hfs in CrF is caused by polarization of that ion by F- such that some s and p character enters the wavefunction.

Acknowledgement The authors would like to thank Professor Anthony Merer for helpful correspondence on the present state of the spectroscopy of the CrH molecule. We are also grateful to the National Science Foundation for support of this research.

References

111 W. Weltner Jr. and R J. kan Zec, Ann Rev. Phys. Chcm.

35 (1984) 291. PI RJ_ van Zee, T-C. DeVore. J.L Wilkerson and W_ Wcltner Jr., J_ Chem. Phys. 69 (1978) 1869, R-J_ van Zee, CM_ Brown and W. Weltncr Jr., Chcm. Phys Letters 64 (1979) 325; A Dendrams, R.J. van Zee and W. Weltner Jr., Astrophys. J. 231 (1979) 632. R J_ van Zee, T C DeVore and W_ Weltner Jr., J_ Chem. Phys_ 71 (1979) 2051_ J.F. Harrison, J. Phys. Chem. 87 (1983) 1323. CA. Baumann, R-J. van Zee and W. Weltner Jr., J.

Chem Phys. 79 (1983) 5272. W.C. Easley and W. Weltner Jr., J. Chem Phys. 52 (1970) 197; LB. Knight Jr. and W. Weltner Jr., J. Chcm. Phys. 54 (1971)

LETTERS [7]

LB.

Knight

8 February Jr. and J. Steadman,

J. Chem.

1985

Phys. 77

(1982) 1750. [!3] R-M_ Gol&ng and WC

Tennant, Mel_ Phys. 28 (1974) 167. [9] C K. Jen, VA. Bowers, E L. Co&ran and S N. Foner, Phys. Rev. 126 (1962) 1749. [lo] W. Wcltncr Jr.. Magnetic atoms and molecules (Van Nostrand, Princeton, 1983). [Ill M-T. Hutch&s, in: Solid state physics, Vol. 16, edsF. Seitz and D. Turnbull (Aademic Press, New York, 1964) pp. 227-273. 1121 3. Kleman and U. Uhler, Can. J. Phys. 37 (1959) 537_ 1131 S O’Connor, Proc. Roy Irixh Agd. 65 (1967) 95; J_ Phys. B2 (1969) 541_ 1141 0. Engvold, H W&l and J.W. Brault, Astron. Astrophys. Suppl. Se1 42 (1980) 209; B. Lindgren and G. Olofsson, Astxon. Astrophys. 84 (1980) 300; R. Clegg and S. Wyckoff, Mon. Not. R_ Astron. Sot. 179 (1977) 417. 1151 G.R. Smith and W. Weltner Jr., J Chem. Phys 62 (1975) 4592. 1161 R.F. Curl, Mol Phys 9 (1965) 585 I171 L-B_ Knight Jr. and W. Wcltner Jr., J. Chem. Phys. 53 (1970)4111. [I83 A.J. Merer, private communication 1191D.L Michalopoulos, ME. Geusic, S.G. Hansen, D.E Powers and R.E. Smalley, J. Phys_ Chem. 86 (1982) 3914; VE. Bondybey and J.H. English, Chem. Phys. Letters 94 (1983) 443. PJI L Brewer and J S Wmn, Faraday Symp. Chem. Sot. 14 (1980) 126. [21] W. Gordy, J. Chem. Phys. 19 (1951) 792; L. Pauling. The nature of the chemicd bond, 3rd Ed. (Cornell Univ. Press, Ithaa, 1960)_ [22] T-C. Dcvore, RJ. van Zee and W. Weltner Jr.. Proceedings of the Symposium on I-l@ Temperature Metal Hahdc Chemlslry, Vol. 78-1, eds. D.L. Hildenbrand and D-D. Cubicdotti (The Elcctrocl~em. Sot. Inc_, 1978) pp. 187-198. 1231 L B. Knight Jr.. W C. Easley and W. Weltner Jr , J. Chem. Phys. 54 (1971) 322. [24] P-H. Kasai, Phys. Rev. Letters 21 (1968) 67.

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