Microwave spectrum of pivalonitrile-N-oxide

‘Received29 June 19?1’

The microwave spectrum

ofpkalonitziie-N-oxide

is shown to be that

of a symmetric

top witfr& = 1459.2270.+

(63) Ml&,DJ = 0.102 f (22) MI& and DJK = 13.151 t (29) kHz. The IOU+ desnerate banding mode was estimated to be at 86 i 20 cm-’ from intensity measurements,

12 and J= 12 -+ 13) of the microwave spectrum of pivalonitrile-Nsxide (PNO) were recorded and measured in the frequency range from 26S to 40.0 GHz, employing a Hewlett-Packard Stark.moduIated spectrometer (modulation frequency: 33 kHz). The fre. quency measurements were carried out using an Sdigit Hewlett-Packard counter. TE,e accuracy of the frequency measurement is limited to about 0.20 MHz due to the considerable width of the absorption fines, wM& is about 1.2 MHzat appr~~ately 10M2 torr. This observed width is ProbabIy caused by a iarge line width parameter due to the large dipole moment (-4 debye) of the PNO, rather than an experimental n&function of ,the instrument. This can be anticipated from the fact that a line width parameter of 94.2 MHz./torr has been foqnd for methylcyanide f 121, IO times the value far methyIacetylene. The observed hne width for PNO is Sor 6 times the fine wldti’obsew&d for (cCH3)3C-_(==c-C% [13-I, so that we may con&de &at the very farge Ene widtfi irnot associated wit&the titemal motion of the pi&o group. Most measurements were carried out at -3O’C in order to pop&&e the ground state energy levels more than the higher vibrational levels, to reduce the line width and to minimize decompos~~o~ of the com-

The observation of the rotational spectrum of p~valonit~Ie-N-ogre, (CH3)~C-C~N+0, represents an extension of recent spectroscopic studies of the monovalent fuhnido-group, -CNO, in various environments. In recent years nitrite oxides (R- CNO) have attracted considerable interest in organic synthesis due to their remarkable’reactivity [I ] _ Their reactions with alkenes and al&es are prototype reactions for the 1,3 dipoIar cycloaddition reactions studied by Huisgen and co-workers [Zj _However, until 1967 very little was known about the georn~t~~~ structure of the fulmido-group. A linear configuration of the molecule HCNU was indicated by Beck and Feldl [3, hf on the basis of ihe infrared spectrum, and was further confumed by,W~nne~s~r and Rodenseh from the microwave spectrum ,151. In a series of communications [5-91 considerable spectrosiopic information has been reported and an rSstrucrure of fulminic acid, HCNO, and deutero fulminic acid DCNO, has been determined. En 1970 tsvo groups independently reported measurements of the microwave spectrum of rn~thy~~e oxide [lo, 111. Fourtransitions(J=9j10,f=10-tll,J=lJ-, ,‘i

.’

‘* W&k supported by the NationaI Science Foundation,,D,C., &d the Mississippi State Uni&ity. f Presented in part at the 26lh Symposium on hfoIec& Structure and Spectroscopy at the Ohio State University, ‘Columbus, 08% Jtie 1971, t Nation4 Science Foui~$tiqn Senior Foreign Scientist klIcw on le$@of absence from the Ins&a&? of Physical ChemiNy, Ur&ers$y of Ki$; Germany; -. ‘.

pound.. : : The sample of PNO was prepared in a three stage synthesis

following essen%aIly the procedure outlined by Zirmer a@ Ciinthe$ iI41 and @xE&YI~~~ flS ] . Zkmples of crude PNO are purified by sub!imation under, reduced pressure dire&y k.sarnple tubes at-

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- Voltime 11, number 4

CHEMICAL,PHYSICS LE’i-TEFS

‘, 1 November 1971

:

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Fig. 1. Recorder trace of the / = 10 + 11 rotational transition of (CH&C-CNO lowest lying bending mode.

tached to the microwave Stark cell at -15’C prior to recording the spectra. The spectrum of the monomer

in excitedvibrational states up to VI= 7 of the

1

I

1 I

PNO can be observed at room temperature for about 5 hours before noticeable loss in line intensity is re-

-_

lai3~-c-c-N-o0;

J.Kl-II.v,~o !

K=O,I 234

5

6

7

8

9

IO

corded. As anticipated from the previous work on HCNO [5-91 and CH,C!O [lo, 111, (CH,),C-CNO shows the spectrum of a symmetric top with a linear arrangement of the hezvy atoms EC-CsN+O. Each rotational transition observed shows two remarkable features: (i) due to the presence of a very low lying bending

mode(u, = 70 to IO0 cm-l j transitions from molecules in excited vibrational states up to yt = 7 can easily be observed even at -35’C; (ii) each transition for vI = 0 to V~= 7 can be resolved into its K-substructure

Fig. 2 shows the re-

solved K-structure for the vibrational ground state, in theJ,= IO+‘11 transition,TheK= 7,8 and9 components

are overlapped

by transitions

of molecules

in

another excited vibrational state. This,K-pattern can ,’ be fitted to the’standard kpkion for the rotational, .,

‘.

.’

state of the J =

of pivalonitrile-Naxide.

frequencies of molecules in a nondegenerate vibrational mode

Y- &!$(J+1)

+ 4DJ(J+1)3

,

- 2DJKcJ+1)K2

(1)

with the selection rules for electric dip,ole transiti&s J + J-:- 1;K + K. The observed and &lculat~d frequencies %Gtht.be-appropriate ass@ment are given in to DJK

table -I, which also lists the’ term prop&ion,@

w&h is responsible for the resolved K-structure. The, calculated: frequencies. were obtained using the follow;

,.

for the ground vibrational

10 --L11 rotational &n&ion

and i-type components.

Fig. 1 shows these unique features for the J= 10 + 11 transition in some detail. To our knowledge this is the first time that for a symmetric top having a QX-&)~Cgroup the K.structure of the individual absorption peBks of rotational lines arising from various vibrational states could be resolved.

GHz Fig. 2. K-structure

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Volume 11, number 4

Obsmvedand caktiated frequencies (I&h) &~ota~t~ A&nknt f=l+l

Observed ” frequency@

.

9410

0 1 2 3 ‘4

11412

12+ 13

32 102.45 32 102.16 32 101.29 32 099.85 32 097.82 32 595.22 32 092.03 32 088.27 32 083.93 32 079.01 32073.52

32101.21 32 100.05 32097.84 32 095.29 091.91 32 087.85 32 083.97 32079.05 32073.60 35020.38b)

; 4

35 35 019.60 018.29 35015.74

i 7 8 9 10 11

35 012.96 009.40 35 005.12 35 000.58 34 995.36 34 989.39 34 982.74

0 1

37 938.51@

3”

37 37 935‘93 937.62

4

6”

:

0.0 -0.32 -1.26 -2.84 -5.05 -7.89 -11.36 -15.47 -20.20 -25.56 -31.56 -38.19

0.02

37939.00 37 938.66 37937.63 37 935.53

-0.01 0.00

37 933.53

-0.08

0.0 -0.34 -1.37

-3.08 -5.47.

-8.55

0.01

37 922.25 37 917.12 37 91i.31 37 904.8~ 37 897.63 37 889.77

:

0.0 -0.29 -1.16 -2.60 4.63 .-7.23 -10.42 -14.B -18.52 -23.43 -28.93

-0.16 0.04 0.18 0.21 0.19

37930.45

37921.99 37 916.98 37911.36 37 904.87 37 897.75 37889.49

-21.30

0.12 0.39 0.05 0.11

37 926.69

7 8 9 10 11 I2

-12.89 -16.83

-0.08 0.20 0.02 0.07 -0.12 -0.42 0.04 0.04 0.08

35 020.74 35 020.42 35 019.48 35 017.90 3s 015.69 35 012.85 35 009.38 35 005.28 3s 000.54 34995.18 34 989.18 34 982.55

37 933.45 37 926.58 37930.46

-0.26 -1.05 -2.37 -4.2L -6.58 -9.47

-0.07 0.09 -0.16 0.02 -0.10 -o.tkJ 0.01’ 0.03

29 171,24 29 167.33 29 162.83

32 101.78b)

0 I

0.0

29 174,66

29 j.74.56

29 171.04 29 167.31 29 162.86

ii 7 8 9 10

in the gmwd ‘stateof (CH&rC-CNO

29 184.13 29183.87 29 183.08 ’ 29181.76 29 179.92 29 177.56

29 183.01 29 181.85 29 179.76 29177.58

0 ‘1 2 3 4

knsitiok CalculateO frequency

29 183.66 b1

ii 7 8 9 10-r 11

I Nqvemkr 197 1

CHEMICAL PHYSICS LETTERS

..

-0.11 -0.26 -0.14 0.05 0.06 0.12 -,0.28

-12.31

--16*75

..

-21.88 -27;70 -34.19 --41.37 -49.24

.‘, ,’

a) Estimated experimental error 0.25 MHZ; itandard dtiiation of the f% is ii16 MI%’ b) Not incfud,e$in the analysis -: ” i

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CHE~KALPHYSICSLE’ITERS Table 2

., .-

Structureof

Assumed structure parameters [S, 7, 10.11,13]

r(C-N)= (CH&zC-

grou;:

1.169A

rK! -H)

= 1.093 A

r(C -C)

=

be considered, but it cannOf resolution mik-neter wave

1,538 A

.LHCC = LCCC Detemhed

L;

= 1.217 A

= 109’28

from ground state rotational

constant

-

Tertiary carbon-fuhtido-group carbon bond: r(C-C=) = 1.458 +-(IO) A

ing constants:

Eo = 1459.2270+

1 ,l November 1971

brdad lines of the widely spread K-sty
(CH&C-C=N-rO

-C=N+_Ogroqp:-&‘O)

..

(63) MHz,

DJ = q.102 a (22) kHz ,

D,,=13:150_1(29)kHz, which were obtained by a least squares fit to the above expression. The standard errors are given in units of the last significant figures. The bond length between the tertiriry carbon and the sp-carbon of the fulmido group is determined to be 1.458 A, under the assump tions given in table 2. A complete t,-structure determination based on the observation of the microwave spectra of the various isotopic species should not , change this value more than +O,O]OA, and will be rem pot-ted elsewhere. This value is somewhat larger than the r’ bond length found for the corresponding bond in CH,-CNO, 1.442 ‘8, which is to be expected upon subs!itution of the CH3 - group by the (CHJ)JC‘group. The effective rotational con&ants for molecules inexcited vibrational states, esp&.Uy for the lowest lying bending mode, differ considerably from those of the ground state, as indicated ir? fig. I. Following the numbering of fundamental vibrational frequencies of molecules of this type established by Hiittner and Zeil [16j, the lowest frequency bending mode is most probably the doubly degenerate =C-(ZN+ bend, of ‘symmetry-species E, &. Torsional satellites, which might.give some indication of the torsionai barrier of the methyl groups, would most likely fall on the low frequency side of the ground state absorption as is the cz& for other tertiary butyl compounds [ 171. The

be decided lulli data on

hfgh PNO becoine

available. The individual line&of which eacS group of vibrational satellites is composed, have not yet been assigned, so that the location of the K = 0 line for vt = v@) = 1,2,3 etc. could only be estimated. The strongest lines in each group, with the applied Stark field of 50 V/cm, shouid be the K = 1 and K = 2 transitions_ We can extract for the different distinct groups of lines mentioned above average effective B,,values which allow us to calculate cer and err values for the e2 mode: cu,= -6.27 (50) MHz, rrr = 4.17 (9) MHz. These approximate values are given mainly to indicate numerically Lie magnitude of the vibration-rotation interaction. Furthermore, fig. I can be used to measure the integrated intensity of the various rotational lines belonging to.each excited vibrational state with E = 0, 1,2 and 3 in order to obtain an vr = v(v12) estimate of.the vibrational frequency. From a comparison of the relztive intensities of the groups of rotational lines, found by integrating the portion of each group which extends above the exuapolated base line, to those expected from the Boltzman distribution,

the fundamental frequency of this bending mode is estimated to 6: 86 k 20 cm-l. The collaboration of Dr. Earl G. Alley and Mr. Douglas Dollar from the Mississippi State Chemical Laboratory in -&e synthesis of pivalonitrile-N-oxide is gratefully acknowledged. I would like to thank Mr. John HensIey for his substantial help’in containing the microwave measurements. The laboratory facilities were kindly made available to the au&or by Dr. Gordon E. Jones, which is gratefully acknowledged. References [l) Ch.Grundmum, Synthesis 7 (1970) 344. i2) R.Huisgen, Angew. Chem. 75 (1?63) 612; R.Huisgen and M.Christl, Angew. Chem. 79 (1967) 471. [;] W.Beck aad X.Feldl, Angew, Chem:78 (1966) 746; W.Fkck and X;Feldl, Angcw. Chem. 5 (1966) 722.

CHEMICAL PHYSICS LETTERS [4]‘W,Beck, P.Swoboda, K.FeIdl and:tiSt.Tobias, Chem. : Ber. 104 (197X) 533. [S] M.Winneu%serand H.K.Bodens&, Z. Naturiorsch, 22a (1967) 1724. [6] H.K.Bodenseh. and M.~innewis&, 2: Naturforsch. 243 (4969) 1966. [7] H,K.Boden&h and M,Winnewisser;Z. Naturfoiscb. 243 (19691 197% [83 M.Winne&r

axtd B.P:Wiie~r,

i. Nntu*brsch

26a (1971) 128. f9j M.Win.newisserand B.P.Winnewisser,3. Mol. Spectry., to be published. [IO] H.K.Bodens?h and RMorgenstem, 2. Naturforsch. 25a (1970) iso.

L ‘November 1971

Ill1 P.~,B~ackbu~ RD.Brown, F.R.Butden,J,G.Crofts and F.R.G&rd, Chem; whys Zetters 7 f:970) LO2. [I21 J.A.Roberts,T.f:.~~lgandChunC,Lin,J.Chem. Phys. 48 (1?68)4046. j13] H,K.Bodens&, R.Gegen.beimer,3;Mennicke and W&ii, ‘2: Natuforsch. 22a (1967) 523. f 141 G.Zinner and H.Giinthei; Angew. C&em. 76 fI964) 440. [ 151 C&Grundnann, in: ysthoden der m-gm.&men cfmnb, -& fhhn and Werl K!eaxg Thksue Verb Stuitg&,

f965) Band X13; Fottscbr. C&em, Fo;sch. 7 11966) 62. ! 161 W.Hiittner and W.Zeil, 2. Naturforsch, 2.53 (1970) 1281. (171 L.J.Nugent, D.E.Mann and tr.R.Lide Jr.. I. C%em.Phys 36 (1962) 965.

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