The microwave spectrum of cyclopentadienyl nickel nitrosyl in an E1 vibrational state, V14 = 1

JOURNAL OF .IIOLECCLARSPECTROSCOPY36,19-60 (1970)

The

Microwave Spectrum of Cyclopentadienyl Nitrosyl in an E, Vibrational State, V,4 A. School

P.

Cox

oj Chemistry,

University

RI. Schuster

AND A.

Laboratory,

Nickel

= 1

H. BRITTAIN’ of Bristol,

Bristol,

England

J. WHITTLE

The University,

Manchester,

England

The J9410, 134 14, and 14-+ 15 rotational transitions of cyclopentadienyl nickel nitrosyl have been studied in successive excited states, V,, = 1-6, of the lowest bending mode. An analysis of the 814 = 1 transition is given using a formula derived for a J+J + 1 transition of an El vibration for symmetric-top molecules of CS, summetry. The assignment of the observed spectrum leads directly to values for B, DJ , and DJK for t,he excited state of 1261.187 + 0.003 MHz, 0.069 f 0.005 kHz and 2.72 f 0.01 kHz, respectively. Also lllJ, the coefficient of t,he diagonal elements of the third order Hamiltonian h3’, is determined to be 8.0 f 0.1 kHz. The transition centers are then used to assign the Z-type doublets (K1 - 1) = 0 lines for each transition, which cannot be assigned a priori since they are partially The coefficient p14 = 0.348 f

of the matrix

elements

obscured (1,K

by neighboring

transitions.

/ h,’ / 1 + 2, K + 2) obtained

is

0.002 MHz which gives a calculat,ed value of r14 = 0.754 + 0.004. I. INTBODUCTIOK

The most common axially symmetric molecules are those with CaVsymmetry and the rotational spectra of these molecules in I3 vibrational states have been extensively studied. Molecules of C,, symmetry are of great theoretical interest and yet have received little attention, so far as rotational spectra in excited vibrational states is concerned, probably because few molecules of this class lend themselves to detailed experimental study. Two types of degenerate vibration, EI and i% , may be studied and interpretation of the rotational spectra of C5, molecules in these excited vibrational states presents features not found in the CBVcase. As shown below, the constants obtained from a fit to the observed spectrum for C5, molecules in El states provide directly values for Bv ,the rota1Present Michigan.

address:

Department

of Chemistry, 49

Michigan

State University,

East Lansing,

50

COS,

BRITTAIN,

AN11 WHITTLE

tional constant in the excited state, arid the constant qlJ , tie coeficient of some diagonal matrix elements of the third order Hamiltonian (1). This contrasts with the situation for CsVmolecules where the constants B* and p* determined by a fit to the equation derived by Grenier-Besson and Amat (2) describing the frequency of transitions J--+J + 1 are related to three unknowns: B, , T[., , and Y, the coefficient of (2, -1) resonance, which a,re not, therefore separately determined. II. FREQUENCY

OF TRANSITIONS

The only difference between the treatment particular off-diagonal elements of hz’ which A rule proposed by Amat (3) requires that Al, K f AK) to be nonzero for a molecule of AK -

InAl = nN

J-4

+

1

of Czv and C;i, molecules lies in the contribute to the energy.? for a matrix element (I3 K / hp’ / 2 f C,, symmetry.

N = 0, fl, f2, E, E, vibrations; 1 m z.zz2 Ef vibrations.

(1)

m =

The only off-diagonal

elements

(1, K ) b’ / 1 f (1, K ( hz’ 11 f

for Csv molecules

of hi able to contribute

are

for El vibrations; for Ez vibrations.

2, K f 2) 2, K =I=1)

In the case of El vibrations (which we deal with henceforth) the matrix element above is the one normally responsible for l-type doubling in the levels (K1 - 1) = 0. The (2, - 1) element disappears for the CSVcase and we thus write for the transition frequencies J-+J + 1 the formula of Grenier-Besson and Amat (2), setting r = 0. v = 2B* (J + 1) -

4D.,(J

+ l)(KI

+ &*(J b4q(J

-

2DAJ

+ 1)” -

+ j14q2(J + 1)3 j(B--++AT)(K1-1)

B* = B, - D,,

if (KI -

1) = 0,

if(KZ-

1) #O,

+ qt., , ‘2q”

P*

This formula Amat :

is subject

=

71tJ -

1)’

1)

+ 1)

J

+ l)(KZ

2DJK+

(B

_

to the same limitations

A

(2)

(3) __

+

(4)

&.).

as that

of Grenier-Besson

2This stat,ement is true assuming we neglect all contributions sixth order of magnitude and assuming J and K are small.

to the energy

and

beyond

;\/IICROWAVE SPECTIEUM OF El MOUE IN C5H5NiN0

.j 1

(a) it is correct insofar as matrix elements contributing to the sixth order are negligible;

energy above

(b) it is correct insofar ing inequality must hold

as perturbation

f$s(J

calculations

+ 1)f << I B -

are valid, i.e., the follow

A + A{ (.

(3)

The quantities B*, p*, D, , D,, , p, [$/(B - A + Al’)] mq be found by :I least squares fitting process. The unknowns in B* and p* are B,, and qlJ only, so that these constants may be calculated directly from the transitions J-+J + 1 in a single vibrationally excited state. A formula for transitions in an 82 state may be obtained by setting Q = 0 in the Grenier-Besson and Amat expression. Here we have the same situation as for Cav molecules, i.e., two determined constants R*, p* and three unknowns B,, ,

9, “. III. EXPERIMENTAL

METHOIX3

Sample preparation and handling of C,H,NiNO are dealt with in Ref. (6). The microwave spectra of the excited states were studied between -20 and 25°C spectrometer. at low pressures ( ~10 Nme2) using a 109 kHz Stark-modulated alechanical sweeping of the klystron sources with recorder techniques and long time constants were utilized to obtain overall spectral patterns (see Figs. 1 and 2). Highly resolved studies and accurate peak freyuency measurements were made using oscilloscope presentation. The peak measurements are reproducible to ho.05 JIHz for the nickel-58 (see Table II) and to ho.1 MHz for the nickel-60 spectrum (see Table IV), though interference from overlapping lines and Stark components could introduce larger systematic errors in some cases. For example the lower I-type doublet (KI - 1) = 0 line for each transition is less accurately det#ermined than its upper partner. IV. THE SPECTRUM

OF CYCLOPENTADIENYL NICKEL El VIBRATIONAL STATE

NITROSYL

IN AN

Prom Iaser Raman studies (J), the lowest vibrational mode in CsHsNiNO has been assigned to the ring -NCS bending mode (El) at 153 cm-l which we desigcorresponds at 25°C to a Boltzmann factor of nate y14 . This wave number (2 X 0.4X = 0.96) for 1’14 = 1 and even for VI4 = 6 predicts a vibrational satellit,e ,lig of the ground-state intensity. This mode gives rise to an almost continuous, rich satellite spectrum for more than 300 hlHz to high frequency of the ground-state microwave spectrum (see Fig. 1). We concern ourselves in this paper &h the rotational spectrum of molecules in the state IT14= 1. The observed transitions J9+10 and J14+15 for I’,,* = 1 are shown in Figs. 3 and 2, respectively. The weaker, unnumbered, lines to the low-frequency end of Fig , 2 arise from the @‘Ni isotopic molecule and are 26 % of the intensity of the

COX,

BRTTTAIN,

H

-r. 0

AXI)

WHITTLE

MICROWAVE

SPECTRUM

OF

El RilOll~ IN CjH&iNO

37855+3 MHz Ki-I=0

37813.76 MHz KI-I

=o

37831.29 MHz /

FIG. 2. Recorder trace J14-+15, number labeling is for the nickel-58

(moderate V II = 1 of CsHjNiNO isotope and refers to Table II.

resolution).

The

corresponding lines in the 58Ni molecule. To the high-frequency side of the lines of Figs. 2 and 3, and not shown here, are a considerable number of intense lines due to molecules in the state VI4 = 2. Because of this considerable overlap of spectra it is not obvious which are the lines (KZ - 1) = 0, The statistical weights of the various vibration-rotation levels as calculated by Wilson’s Method (5) are shown in Table I; and as shown, the levels (Kt - 1) = 0 have a weight 35 that of most other levels and $5 that of levels having (Kl - 1) a multiple of 5. Also the splitting of these levels makes it more difficult to Stark-modulate these lines completely making a positive identification of these lines against the background of C5H560NiN0 lines and VIA = 2 lines difficult.

COX,

54

BIZITTAIN,

AND

WHITTLE

MHZ

4 25 222.11MHz

FIG. Table

3. Oscilloscope

tracing

J9*10,

V II = 1 of CsHS5*NiN0.

The

labeling

refers

II.

STITISTIC,\L WEIGHTS

OF ROT.IYTONAL

Lml:ts IN CjH;NiNO Vibration

8

12

12

16

24

24

K=5p*l

12

28

24

h- = 5p zt 2

12

24

28

K = 0 K = 5~” (K # 0)

KL -

1 1

-2

2 2

-3

3 3

-4

4 4

-5

5 5 6

-7

7

-8 “’

Species of S

0

8, 8 12

1

12 12

E1 ix,

2

12 12

E? E’1

3

12 16

E? 2A

4 5

12 12 16

E, El 2.4

12 12

E2

6

-6

6 7

Stat. wt 12

-1

0

--ap = 0, fl, f2,

1

12

A, A E?

E E:

to

MICROWAVE

RPECTRUhI

OF

El MOI)P: IN CiH,NiNO

55

111 the absence of these lines and hence a center to the I’,, = 1 pattern it is not possible to adopt the conventional assignment procedure as used by GrenierBesson and Amat (2). The method of assignment used was essentially one of trial and error using intensities as a guide. Two criteria used to test any trinl assignment were (a) The assignment must produce values for D IK and D, similar to the ground state values (6). (b) The assignment must predict a considerable overlap of lines at 37 531.3 and 25 222.1 MHz to produce the dominant large peak in each transition. intensity from 1 to 5 In Fig. 2, the series of lines 1, 2, 3, 4, 5 show diminishing (with the exception of 4) which suggest a series with K increasing from 1 to 5. The intensity of line 4 suggests that (KI - 1) = 5 for thii line and the series 1 to 5 corresponds to (KZ - 1) = 2, 3, 4, 5, 6, respectively. The sign of (KZ - 1) applying was ultimately determined from the relative intensity of lines 16 and 17 once these had been assigned. The assignment XL* substantiated by observation of the lines KZ - 1 = -9, -10 in the transition J9--+10 (numbers 6 and 7 in E’ig. 3). The lines KZ - 1 = - 14. - 1.5 in the transition .I1 4-1.5 were obscured by the C&HPNiNO spectrum. Thfh lines 1 to 5 wertl fitted to :\II equation of form

v = (k’/ ?--+ _ 1)

h + c( KI -

1) + d(Kl

-

l)“,

and the values of a, b, c, d were used to predict further lines which were then included in the fit and the process was continued until all lines in the J14--tl5 transition were &ssigned. Use of the ground-state value of D,(6) then permitted in assignment of the transitions J9-+10 and Jl3 414. Use of these frequencies the full Eq. (Lz) then tbnabled calculation of a value for D, in the excited state. The observed frequencies, their assignment and the frequencies calculated using Eq. (2) are given in Table II. The lines (K1 - 1) = 0 in each transition were found by searching for lines equidistant to either side of the transition centers computed from V, =

2R*(J

+

I) -

4D;(J

+ 1)3.

(7)

The molecular constants derived from the fit are given in Table III. The value of [Is was calculated using a valuca for d of 4397 1IHz (7) and (Ye, was calculated using a value of 1259.2Sl (=tO.O06) JIHz for the ground state Ra(@. The value of ql., given above is that for the coefficient of the matrix elements (I, K j ht’ 1 I f 2, K f 2) and is four times smaller than the Z-type doubling constant used commonly by other authors (8) which is sometimes designated P. P is 1.392 MHz, a factor of two larger than the quantity ~R,?/v~~ = 0.691 MHz. This represents a somewhat largSr%rdrviat,ion between I’ and 3B0z/v~.~ than has been found for Cap molecules.

56

COX, BRITTAIN, ANI) WHITTLP:

TABLE II No.

ObservedFreq.(MHz)

CalculatedFrea.(MHz)

J9 + 10

(KP,- 1)

25 237.39

0

25 237.48

25 209.73

0

25 209.64

25 226.15

-1

25 226.06

25 224.44

-2

25 224.50

25 223.68

-3 -4

25 223.71

25 223.04 25 222.11

3

25 222.43

4

25 222.35

-5

25 222.33

sh

5

Assignment

25 220.76

25 223.03

2

25 222.19

5

25 222.07

6

25 221.63

-6

25 221.57

7

25 221.07

1

25 220.96

-7 8

25 220.37

25 220.72

sh

-8

25 219.78

6

25 218.76

-9

25 218.74

7

25 217.58

-10

25 217.60

513 + 14 35 332.04

0

35 293.04

0

35 293.13

35 319.62

-1

35 319.63

35 315.78

-2

35 315.69

35 314.02

-3 -4

35 314.00

-5 -6

35 311.60

35 312.78 35 311.58 35 310.39

35 332.11

35 312.76 35 310.41

MICROWAVE

SPECTRUM

OF El MODP: IN C&HsNiNO

.1,i

TAEXS II (Continued) NO.

ObservedFreq.fMHz)

Assignment

513 + 14fCont.)

(K% - 1)

35 309.72

35 309.16

CalculatedFreq.fMIiz)

4

35 310.04

3

35 309.86

5

35 309.82

6

35 309.33

-7

35 309.14

2

35 308.93

7 -8

35 308.62

8

35 307.71

35 306.62

9

35 306.62

35 306.25

-9

35 306.26

35 308.65 35 307.73

35 305.34 35 304.61 sh 35 302.80

35 307.76

1

35 305.44

10

35 305.37

-10

35 304.62

11

35 303.94

-11

35 302.84

12

35 302.35

35 300.96

-12

35 300.92

35 298.84

-13

35 298.86

35 296.71

-14

35 296.65

sh

514 -c15 37 855.68

0

37 855.71

37 813.76

0

37 813.95

37 843.24

-1

37 843.49

37 838.81

-2

37 838.69

2

37 836.77

-3

37 836.69

3

37 835.28

-4

37 835.26

4

37 833.98

37 833.97

5

37 832.63

-5 -6

1

37 832.66

58

COX,

BRITTAIN,

AND

WHITTLE

TABLE II (Continued)

No.

Observed Freq.(MHz) 514 + 15 (Cont.)

6

37 831.29

Assignment (Kt

-

Calculated Freq.(MHz)

1)

4

37 831.77

5

37 831.60

3

37 831.49

-7 6

37 831.27 37 831.11

7

37 830.26

7

37 830.38

2

37 830.30

8

37 829.86

-8

37 829.77

9 10

37 829.44

8

37 829.43

9

37 828.28

-9

37 828.14

37 826.99

10

37 826.94

13

37 826.38

-10

37 826.37

14

37 826.01

1

37 826.00

15

37 825.49

11

37 825.43

16

37 824.43

-11

37 824.46

17

37 823.97

12

37 823.73

18

37 822.31

-12

37 822.39

19

37 821.85

13

37 821.86

20

37 820.23

-13

37 820.18

21

37 817.84

-14

37 817.80

22

37 815.23

-15

37 815.28

11 I2

37 828.20

Lines indicated by sh are observed, unresolved shoulders on larger lines.

Table II shows that agreement between observed and calculated frequencies is very good, fully justifying the assumptions made in the derivation and application of Eq. (2). The necessity to include third order diagonal terms is seen directly here in the well-determined value obtained for ~1~. The values for 01, and 0,” are close to the ground-state values for D JK and D, of 2.66 (3~0.01) kHz and 0.078 (~0.02) kHz, respectively, obtained by Cox and Brittain (6)) the difference in DJK probably represents a real shift of this constant with vibrational state.

MICROWAVII:

SPECTRUM

OF EI MOJII~: IN CjH:,NiNO

TABLE

III

M~LECUL~IR CONSI~.~NTSOF C$H:‘NiNO Value

E:lXX

1261 .I92

0.005 0.05 0.01

3.91 2.72 0 O(i9

0 .003 0.003 0.1 0.001 0.003 0.01

‘,I,

(MHZ,

0.348

,I,,

ikHZJ

8.0

ill f,‘,. (MHZ) a,4 (hIHz)

0.005

0 ,656

0.751 1261.187 -1.91

TABLK

IV

Jl4 -+ 15, VI4 = 1 TRANSITION FOR C,H?NiNO Observed frequency (MHz) _~~~~_ ()t)scllred b?; 58Ni 3T iSi .9 Ot)fnu+ed by 5*Ni ()t)e~rlred hy 58Ni a7 819.90 37 818.44 37 817 19 37 815.80

37 814.76

Assignment .-.-_ (Kl - 1) 0 0

-7 6 7 2

37 813.07 sh

-8

37 811.45

-9 10 -10

37 809.82

37 813.02 37 812.60

8 9

a Calculated from 5*Ni constants with isotopic responding to a decrease in B* of 0.561 MHz.

_

37 838.84 37 796.92 37 826.40 87 821.97 37 819.98 37 818.44 37 817.11 37 815.79 37 814.93 37 813 .i(i 37 814. fi.5 37 814.J3 :37 81-l. 27 37 813.42

-1 -_2 - 3 --1 -5 -_(i 4 5 3

sh

~__~.

Calculated frequencva (MHz)

37 811.36 37 810.15 37 809.54 frequency

shift

of -16.84

MHz

cor-

COX,

60

BRITTAIN,

AND

WHITTLE

The nickel-60 data given in Table IV shows no significant changes in the vibrational constants from the nickel-58 apart from the isotopic shift of 0.561 MHz in B*. This isotopic shift is larger than that in the ground state (6) and corresponds to the nickel atom being 0.008 A further away from the center of gravity in the excited state. This is probably to compensate for the shortened average projection of the light nitrosyl group on the top axis, due to its large (relative to the ring) vibrational amplitude. ACKNOWLEDGMENT One of us (A.B.) thanks the Science Research Council for a Research authors thank Dr. J. G. Baker for his interest in this work. RECEIVED:

November

28,1969 REFERENCES

1. S. Mass, J. Mol. Spectrosc. 9, 204 (1962). 2. M. L. GRENIER-BESSON AND G. AMAT, J. Mol. Spectrosc. 8, 22 (1962). 8. G. AMAT, C. R. Acad. Sci. 260, 1439 (1960). 4. I. J. HYAMS AND E. R. LIPPINCOTT, Nature (London) 214, 267 (1967). 5. E. B. WILSON, JR., J. Chem. Phys. 3, 276 (1935). 6. A. P. Cox AND A. H. BRITT~IN Trans. Faraday Sot., 66, 557 (1970). 7. C. ROBERTS AND A. P. Cox, to be published. 8. G. G. WEBER, J. IlIol. Spectrosc. 10, 321 (1963).

St~udentship. The