Rotation-inversion spectrum of monodeutero isocyanamide, HDNNC

JOURNAL OF MOLECULAR SPECIROSCOPY

120,28-48 (1986)

Rotation-Inversion Spectrum of Monodeutero Isocyanamide, HDNNC MANFREDWINNEWISSERANDJORGENREINSTAEDTLER Physikalisch-Chemisches Institut der Justus-Liebig-Universitiit. Heinrich-Buff-Ring 58, D-6300 Giessen, West Germany The rotation-inversion spectrum of the isotopic-ally substituted species of isocyanamide HDNNC was observed in the region from 100 to 400 GHz. The 173 observed transitions arising from the inversion states O+and O- were assigned and analyzed using a Watson Hamiltonian (Sreduction). The two Hamiltonians are coupled by an u-type Coriolis resonance (AK. = 0) allowed by the C, molecular symmetry of HDNNC. The off-diagonal matrix elements of this resonance, which are proportional to K,, with the coefficient R,,, were included in the fitting program. The rotational constants found arc:

A B C

0+ State

O- State

196 324.84 (49) MHz 10 243.45800 (58) MHz 9 909.53965 (47) MHz

196 301.96 (64) MHz 10 243.29170 (56) MHz 9 909.53972 (44) MHz

Centrifugal distortion constants were also determined. The inversion splitting between the O+ and O- states was determined to be EbV = 352 1.30(53) MHz and the Coriolis resonance coefficient was found to be R. = 2O4.12(13) MHz. The nuclear quadrupole hypcrfine structure induced by the two nitrogen nuclei was analyzed and the coupling constants were determined. The nuclear quadrupole coupling constants of the normal species, H2NNC, were reanalyzed. Improvements in the spectrometer software since the earlier work made it possible to analyze large numbers of rotation-inversion lines exhibiting nuclear quadrupole inkX&iOnS. 0 1986 AcademicRess, Inc. I. INTRODUCTION

In continuation of our investigations of the high-resolution gas phase spectra of the different isomers of cyanamide, H$JNC (l-6), we have studied the first isotopomer of isocyanamide, namely HDNNC. The millimeter-wave spectrum of the parent species, H2NNC, was observed and analyzed by SchXer et al. (7,8). The inversion splitting between the states O+and O- was found to be 0.369 cm-’ which is indicative of a high barrier to planarity for this pyramidal molecule (8). As can be seen from Fig. 1, isocyanamide is a slightly asymmetric prolate top molecule (qrH x -0.9983, KHD = -0.9965). As in cyanamide the configuration is nonplanar. The barrier to planarity of the NH2 group is low enough so that the effect of inversion of the NH2 group with respect to the heavy NNC group is observable. In a recent communication Jensen and Winnewisser (9) have used this data and calculated the lowest four inversion energies of isocyanamide employing the semirigid invertor Hamiltonian developed by Jensen and Bunker (20). The calculation was based on an ab initio inversion potential function 0022-2852186 $3.00 Copyright 0 1986 by.Acadcmk Ress, Inc. All rightsof reproductionin any form x-cservcd.

28

ROTATION-INVERSION

H2NNC

:

SPECTRUM OF HDNNC x.b *

OH2

29

50pm 77

\

::b HDNNC

OH

\ +-_____ *=(1

0

_______ N F

L Y:N

o--C

I

/-

FIG. I. Structures of isocyanamide (HJWJC and HDNNC) in the principle axis system predicted from ab initio calculations by Vincent and Dykstra (18). The position vectors of the permanent electric dipole moments are also indicated.

calculated by Dykstra and Jasien (I 2) which was refined by fitting to the experimental data (8). The results of these calculations yielded the potential function I’,, = -4313.7~’

+ 2226~~ + 30.9$,

(1)

where p is the out-of-plane angle expressed in radians (see Fig. 1). The potential coefficients are given in cm-‘. The inversion energy for the O- state was calculated to be 0.37 cm-’ relative to the O+state and for the If and I- states 874.07 and 892.07 cm-‘, respectively, with a barrier to planarity of 2070 cm-i. This result of course contrasts with the inversion behavior of HzNCN which exhibits a low barrier to planarity of 5 10 cm-’ and an inversion splitting between the O+and O- states of 49.279 cm-’ using a semirigid bender Hamiltonian (12-14). Independently of the work by Brown et al. (12-Z4), Read et al. (15) measured and analyzed the millimeter- and submillimeterwave rotation-inversion spectrum of cyanamide and arrived at similar results. The separation of the Of and O- states in HDNNC should become even smaller than in H*NNC. One should expect, as for the normal species, intrasystem a-type transitions, and intersystem c-type transitions connecting rotational states of the O+ system with those of the O- system, resulting in a rotation-inversion spectrum.

30

WINNEWISSER AND REINSTAEDTLER

Concerning the molecular structure of H$JNC and its chemical stability relative to other isomers there have been several ab initio calculations (16-20). By comparing the rotational constants calculated from ab initio structures with the experimentally obtained values and the shifts induced by the deuterium substitution, we have found that the molecular structure given by Vincent and Dykstra (18) is closest to the experimental observations. However, to obtain a complete substitution structure for H*NNC, it will be necessary to complete the measurement of further isotopically substituted species. II. EXPERIMENTAL (I)

PROCEDURES

Chemical Preparation of HDNNC

Following the method of our preparation of H2NNC (7, 8) the lithium salt of diazomethane [HNNC]-Lif was hydrolyzed with a slightly acid buffer solution of KDzPO4 in D20 yielding mainly isocyanamide-d, . However, in contrast to the earlier work, the lithium salt of diazomethane was prepared using directly diazomethane and methyl lithium as described by Eistert et al. (21). After mixing 100 cm3 of ether at a temperature of -2°C with a 40% solution of potassium hydroxide, KOH, 15 g Nnitroso-N-methylurea were added while the entire solution was stirred: H2NCON(NO)CH3 + KOH = [H2NCOO]- K+ + HzCNz + H20. After decanting and drying with solid potassium a solution of pure diazomethane in ether. This was added slowly to an ether solution of methyl of - 15°C. The reaction was carried out under

hydroxide pellets for 2 hr, one obtains solution kept at a temperature of 0°C lithium, CH3Li, kept at a temperature a dry Nz gas atmosphere:

HzCNz + LiCH3 = [HCN*]- Li+ + CH4. To this suspension of the lithium salt of diazomethane, we added under continuous stirring 13 cm3 of a saturated potassium dideuterophosphat solution in DzO. The ether solution was then separated from the aqueous solution by decanting and dried over P40r0. Further purification of HDNNC was carried out by removing the solvent and the volatile by-products at a temperature of -30°C until the vapor pressure was down to 0.3 Pa: [HCN2]- Li+ + D+ = HDNNC + Lif. About 10 min are required to vaporize isocyanamide to a pressure of about 1 to 2 Pa in the 16-liter glass absorption cell, when the sample storage vessel is kept at -30°C. Under these conditions the half-life of isocyanamide was found to be 10 min at room temperature and a pressure of 1 Pa. From the recorded spectra we were able to identify not only monodeuterated isocyanamide, but also the dideuterated and normal species. Since the normal species H2NNC appeared instantaneously when we evaporated the monodeutero isocyanamide into the cell, we concluded that isocyanamide is able to easily exchange its hydrogen atoms by wall collisions. By flushing the cell with water vapor having a H:D ratio of 50:50, we were able to achieve the approximate isotope substitution ratios HH:HD: DD = 25:50:25. Therefore, we always had a mixture of three isotopic species in the absorption cell, which made the assignment of weak lines rather difficult.

ROTATION-INVERSION

SPECTRUM OF HDNNC

31

(2) Submillimeter- Wave Spectrometer The spectrometer used for the present measurements is described in detail elsewhere (22, 23). The characteristics of the spectrometer are: l frequency range 100-500 GHz, or for strong lines up to 800 GHz; l Gordy-type frequency multiplication technique, point contact Zener diode, and frequency stabilized Ok&klystrons from 50 to 85 GHz as radiation sources; l InSb detector operating at 1.7 K, l minimal absorption coefficient 3 X lop8 cm-’ at 250 GHz, using a 2-m glass absorption cell with a diameter of 10 cm; l absolute frequency accuracy 30 kHz over the entire frequency range. Over the last few years we have improved mainly the data acquisition and processing system (24). Using a data averager (PAR 4202) and a personal computer (Apple IIe) with a specially built interface, we were able to transfer the 256 point spectra, together with some additional information collected by the dedicated personal computer, from the spectrometer to a main frame computer (CDC Cyber 680). Using this computer, several evaluation and fitting programs could be implemented to analyze the spectra; e.g., a nuclear quadrupole fitting program for two nuclei was used in the analysis of the quadrupole hyperlme structure presented later. III. THEORETICAL

CONSIDERATIONS

HDNNC showed the typical pattern of a slightly asymmetric prolate top rotor with almost equidistant K, = 0 lines for successive J values centered within the K, = 1 doublets. These intrasystem transitions involve the inversion-invariant component pL,of the electric dipole moment. The inversion-variant component pe of the electric dipole moment is involved in the c-type intersystem transitions which are also shown in the Fortrat diagrams given in Fig. 2 for HDNNC and in Fig. 3 for H2NNC. From the comparison of the two Fortrat diagrams it can be seen that the substitution of one hydrogen by deuterium drastically alters the rotation-inversion spectrum: The band center of the ‘QObranches is shifted from 272 to 186 GHz, while the inversion splitting has decreased from 11.1 to 3.5 GHz. In contrast to the transitions observed for the parent species, H&NC, lines arising from HDNNC exhibit no nuclear spin statistics. (1) Molecular Symmetry and Coriolis Resonance In the rigid nonplanar monodeutero isocyanamide molecule, as it is shown in Fig. 1, only the identity symmetry operation is feasible. Due to the finite height of the barrier to planarity, the probability of inversion of the NHD group is nonzero and so the symmetry operation of inversion, i.e., the change of the signs for all coordinates, is also feasible. This symmetry operation may be shown to be equivalent to the point group operation of reflection in the xz plane. According to Bunker (25) HDNNC thus belongs to the C,(M) symmetry group. It was shown earlier (7,8) that the normal species H$INC belongs to the &(M) symmetry group. In order to find out which energy levels of the two lowest inversion states O+ and O- are allowed to influence each other through a Coriolis resonance, we consider the symmetry of the wavefunctions of the energy eigenvalues. Matrix elements of the

32

WINNEWISSER AND REINSTAEDTLER

J

40

:

35

‘:

30

:

25

‘1

20

:

15

‘1

10 5 0

0

50

100

150

200

250

300

350

400 450 frequency /Gffz

FIG. 2. Part of the Fortrat diagram of HDNNC. The a-type intrasystem transitions of the qQi and the qR branches are plotted as squares and the c-type intersystem transitions of the ‘Pa, ‘Q, and ‘&, branches are drawn as rhombs. The band center of these c-type transitions is located at 186 GHz. Solid symbols represent measured lines while the position of the open symbols were calculated. A centered dot indicates incomplete or approximate measurements. In the u-type qRbranch only the K, = 1 transitions are shown.

25

FIG. 3. Part of the Fortrat diagram of HrNNC. The u-type intrasystem transitions of the qQ, and the qR branches are plotted as squares and the c-type intersystem transitions of the ‘PO,Q,, and ‘& branches are drawn as rhombs. The band center of these c-type transitions is located at 272 GHz. Solid symbols represent measured lines while the position of the open symbols were calculated. A centered dot indicates incomplete or approximate measurements. In the u-type qR branch only the K. = 1 transitions are shown.

ROTATION-INVERSION

33

SPECTRUM OF HDNNC

Hamiltonian which are nonzero can only be formed by wavefunctions total symmetry: (!I’(Hl\k’) # 0 * I’(*) = I’(*‘).

of the same (2)

The total wavefunction is composed of the rotational wavefunction, the wavefunction for the inversion motion, and the other degrees of freedom which remain unchanged for all energy levels involved in the observed transitions. Therefore these parts of the total wavefunction are neglected in the following symmetry considerations: k = *rot * !I&. Thus the irreducible representation IV)

(3)

of the wavefunction is = IV,)

(4)

* JWid

Papousek and Spirko (26) showed that all the inversion states *l+(n) (n = 0, 1,2, * * - ) exhibit A’ symmetry, while the !I-(@ (n = 0, 1,2, - - - ) states belong to the A” symmetry species. Table I shows the irreducible representations of the rotation and inversion wavefunctions. Concerning the symmetry of the rotational wavefunctions for the Wang matrices, the reader is referred to Gordy and Cook (27). From Table I, we can deduce the following selection rules for energy levels that could affect each other by Coriolis resonance: (1)

Au = odd,

AJ = 0,

AK, = odd,

AK, = odd

(2)

Av = odd,

AJ=O,

AK, = even,

AK, = odd.

Only resonances of the second type, so called u-type Coriolis resonances, were observable in the recorded spectra of monodeutero &cyanamide. The connecting matrix element of the Hamiltonian is (J, K,, Kc, v[HIJ, K,, Kc f 1, v + 1) = R,K,.

(5)

In order to introduce the molecular constant R, into the least-squares fitting program to reproduce the experimental results, the entire structure of the fitting program had to be altered. (2) The Hamiltonian The Hamiltonian which was used in the analysis of the measured transition frequencies consists of four parts: (1) the pure inversion Hamiltonian Hi,, (2) Watson’s S-reduced Hamiltonian for the O+ inversion state H&O+), (3) Watson’s L&reduced Hamiltonian for the O- inversion state H&O-), and (4) the Coriolis resonance term TABLE 1 Possible Irreducible Representations of the Rotation-Inversion

Wavefunctions for C, Symmetry

34

WINNEWISSER AND REINSTAEDTLER TABLE II Observed and Calculated Transition Frequencies of Monodeutero Isocyanamide, HDNNC

LYl8l -

16, 191 0,191 201 0.20,

-

17, ,8( L9,

0;17, 0.18, 0.19,

119905.9611 159865.8235 119843.9196

199866.8942 119842.8531

219792.3195

219191.1999

*59132.0,09 279597.8682

259731.1991 279 696.9158

3196*0.41*6 339516.8408 3,91,9.,0**

279697.9212 (56, 299660.1910 (55, 319620.4961199, 339 576.8461151, 359 929.6481 ,631 379478.1124175, 199429.8927191,

0.0218 -0.5,55

L 0

0.0110

L

0.0231

I

3196,9.2,,, 339 576.9341 379476.6916

60 946.,653,136, 61 162.8656,1712, 121 909.,*ol

223459.3869 243,6,.3082 264 06L.lW6 284 J6*.9,,4 321918.996, 345236.0080 985 798.6002

120895.9958 L6L 181.9799 I** 33L.9167

16,1*9.*400 181 333.3171 221616.3710

221614.9986

261891.3496 *8*ll*5.3849

261890.63cl4

322 289.1591 342 410.6749

322 289.8395 342 421.0672 382 108.1695 102 932.5332

120 912.1313

69 966.cmO ,,a*, 8005,.5,0, ,,IOI, 99195.6961 ,*0*9, 119919.808L 12291 139890.0329 (811 159866.8324162, 1,9843.,*.99,57; 199818.5715 (57, 219791.18291961 239762.1321 (55, *59731.,110 193,

-0.029,

1

3,9111.*573,78~ ,991**.3**0,96,

-0.599,

0

ROTATION-INVERSION

SPEDRUM

35

OF HDNNC

TABLE II-Continued

0.0146 o.oo*l

1 1

-0.0032 -0.0055 -0.0041 -0.0401

1 L 1 I

-0.0196 -0.1395

1 0

-2.ZL23 -9.9461

0 0

0.0214

0.5

0.0482 0.0913

0.5 0

0.0295 0.0619

0.5 0

0.0321

1

0.0180

1

0.0236 -0.0010

1 1

-0.0019

0.5

0.2128 0.0110

0 1

0.0197 0.0136

I I

-0.0234 0.0266

, I

-0.0302 0.0012

1 1

0.0019

0.5

-0.0231

0.6

-0.0395 -0.0695

0.5 0

-0.0582 -0.0928

0.5 0

-0.0016

1

-0.0168

1

0.0068 0.0110

1

1

1

-0.0315 -0.0015

0.0371 0.0512

1 1

-0.2261 0.0606

0 1

O.OZLS -0.0258

1 1

0.0207 0.0031

1 1

-0.0189 -0.0400

L 1

0.0119

1

-O.OPOS

1

0.026, 0.0036

1

-0.0315 -0.0282

1

0.0130 0.0283

1

1

1

-0.0104 -0.0335

0.0101

1

-0.0539

,

0.0215 O.O,SI

I 1

-0.0158 -0.0521

L 1

0.0315

1

-0.0429

I

0.0486

I

0.0521

,

-0.0129 -0.0487

1 1

9.5417

0

0.0801 0.0808

, I

I

1

1

1

WINNEWISSER AND REINSTAEDTLER TABLE II-Continued c-type 0-- 0’

c-type o+.-oFreg”e”c,er ,“HZ

abr.-ca,c.

abrerred CalC”l.,Std.D~“.,,hl”Z Yt.

Frequencler /II”1

c.bl.-CaIC.

obrerred C.lC”I.,Std.Der.I/“HZ Yt.

2,3018.0987 2*,,40.5,**

Hmh, connecting the two pure rotational Hamiltonians. Furthermore centrifugal distortion constants up to the eighth order had to be included in order to fit the lines with high K, values. The Hamiltonian may be given as (28) @A = H&O+)

(6)

IX& = A,~O-) + Ei”, a-o-

= firoin”

(7)

ROTATION-INVERSION

31

SPECTRUM OF HDNNC K, = 2

E hc /cm-l

0+

0-

211.1 J=23 210.6

I

195.6 J = 22

J=21

3=20 166.3

J= 19

140.31

140.01

FIG. 4. Part of the energy level diagram of HDNNC. The small ax&s on the energy levels symbolize their shift due to the Coriolis resonance observed in the spectra. The irrtrasystem transitions indicated by long arrows were used in Fig. 5 to illustrate the intluence of the resonance on the transition frequencies.

H,

= l/2@ + C)P’ + {A - [(B + C)/2]}P; - DJp

- &pP:

- D&

+ [(B - C)/4 + d,P2](p: + P?) + d*(p4++ P!) + H&E + &Jp*E

- J&Fe

- LKJP*e,

(J, Kcz,Kc,O+IHroinvIJ~ Kz,Kcf l,O-) = R&a,

(8) (9)

where A, B, C are the rotational constants, and DJ, DJK, DK, dl, d2 are the quartic centrifugal distortion constants. The quantities HJK and HKJ are sextic centrifugal distortion constants and the LJK and Lm are centrifugal distortion constants of eighth order. Ra is the Coriolis resonance coupling constant. P, P,, P,, and P, are the operators for the total angular momentum and its components, respectively. The ladder operators aregivenbyp, =P,kI’P,. IV. ANALYSIS OF THE MMW DATA OF HDNNC

As mentioned above the rotation-inversion spectrum consists of u-type intrasystem and a c-type intersystem transitions. In terms of selection rules, this can be written: AUinv= 0,

AJ=O,fl,

AK, = 0,

AK,= fl

AL+,,“=1,

AJ=O,kl,

AK,=rtl,

AK, = 0.

38

WINNEWISSER AND RJZINSTAEDTLER

ot

I 1.

7 I.

I.

I.

I.

I.

14

16

18

20

22

I.

24

I.

26

J"

FIG. 5. Effect of the Coriolis resonance on the u-type qR2-branch Of intrasystem transitions. The difference between the calculated frequencies allowing for Coriolis resonance (R. # 0) and ignoring the resonance (R.= 0)is plotted versus the rotational quantum number J. Again solid symbols represent observed transitions, while the dotted symbols indicate uncertain assignments. The O- intrasystem transitions show the same deviations with opposite sign.

Figures 2 and 3 give an overview of the dominant series in the spectrum for the HDNNC species and the parent species. For clarity the high K, transitions allowed in the frequency range shown have not been entered. The u-type intrasystem transitions are indicated by squares (Kl), while the c-type intersystem transitions are plotted using rhombs (0). The solid symbols represent measured transitions, while the positions of the open symbols were calculated using the adjusted molecular constants listed in Table II and the Hamiltonian presented in Section III. Symbols with a centered dot indicate transitions which were measured and assigned, but were not included in the fit due to resonances which were not analyzed, or which were not measured for all K, values (in the u-type R branch). (1) a-Type Spectrum The most intense lines in the spectrum belong to the u-type R branch. The assignment of those lines was, with some exceptions, straightforward. All transitions appeared as close doublets in the spectra, due to the small difference of the effective rotation constants in the two inversion states O+ and O-. The intensity ratio of the doublets is 1: 1 in contrast to the normal species (7, 8), due to the absence of nuclear spin statistics. The frequency interval between the doublet lines is smaller in monodeutero isocy-

ROTATION-INVERSION

39

SPECTRUM OF HDNNC

TABLE III Parameters of Isocyanamide (HDNNC and H2NNC) for Watson’s S-Reduced Hamiltonian in the Y-Axis Representation Parameter

HDNNC

H2NNCa 0-

0+

A

/MHz

196

6

/MHZ

10

C

/MHz

324.84(49)b 243.458

196

OOL58)

9 909.539

10

68147)

301.96(64) 243.291

9 909.539

4.625

282

686.50120.00)

70(56)

10

761.522

23(33)

72144)

10

525.292

19127)

831891

DJ

/kHz

DJK

/kHz

OK

/MHZ

dl

/Hz

-223.49(23)

d2

/Hz

-50.38(69)

5.269

/HZ /HZ

LJK

/Hz

LKJ

/Hz

R,

/MHz

451150)

421.79(12)

316.89(12)

214.OOl18.00)

55.63(181

-176.04(10) -34.34166)

1.74(21)

HJK HKJ

Einv

averaged

0.90(33)

-272.6t7.5)

-63O.Ol20.0) -0.304135)

-0.055(10) 4.17(13)

22.7t1.3)

204.12(13) 3

/MHZ

Number

of

Data

(Std.

dev.

of

a)

From

b)

Numbers

ref. in

521.30(53)

11

173 fit)

/kHz

061.80(9.20)

240

29.3

25.0

(8). parentheses

are

standard

deviations

in

units

of

the

last

digits.

anamide than in H*NNC. This is indicative of the fact that the two inversion states O+ and O- are lower in energy and thus closer together than in HJVNC. The asymmetry splitting of the K, = 1,2,3 transitions and the nuclear quadrupole hype&e structure of transitions with small K, were used to confirm the assignment of the lines observed unambiguously. For some of the low Jtransitions the quadrupole components of the inversion doublet lines overlapped and a detailed calculation had to be carried out to evaluate the coupling constants and the unsplit line positions. (2) c-Type Spectrum The quadrupole hyperfine structure of the c-type transitions was extrapolated from that observed in the normal species H&NC (8, 23). We were able to assign two series of transitions which showed the typical splitting and almost constant 2:l intensity ratio, which was also observed for the parent species (see Figs. 6d and 7d). These transitions had to be the two ‘Qo branches. The J assignment was made by comparing

40

WINNEWBSER

AND REINSTAEDTLER

a

b

I.5 3 i 5 2

I.0

I.0

.5

.5

% L

p

;: ; ”

<

0.

0.

-.5 -.5

161064.5

161065

161665.5 hqumcy

161666 Mfz

161065.5

161067

161133.5

161134.0 161134.5 161135.0 rrequncy m

161135.5

c ;

2.0

2.0

1.5

f E i L

1.n

I.0

0.

0.

% 5 2 ; ”

-.5

-1.U

126652

126653

126654

126655

FrG. 6. Typical quadrupole hyperhne structures of transitions of HDNNC. The upper traces were observed, while the lower traces were calculated using a least-squares fitting program. (a)-(c) each show two transitions with overlapping quadrupole structures. The numbering (a) to (d) of the figures corresponds to the lineshape types quoted in Table N: Fig.

Transition

Unperturbed frequency (MHz)

8 (5, 4,0+)- 7 (5, 3,0+) 8 (5, 4,0-)- 7 (5, 3,0-) 8 (4, 5,0+)- 7 (4, 4,0+) 8 (4, 5,0-)- 7 (4, 4,0-) 6 (4, 3,0+)- 5 (4, 2,0+) 6 (4, 3,0-)- 5 (4, 2,0-) 33 (1,33,0_)-33 (0,33,0+)

161 086.2461 161085.0937 161 135.1745 161 133.9509 120 853.6136 120 852.6761 113 251.0918

the predicted and the measured frequency differences between adjacent transitions of different J. The final confirmation of this assignment was achieved by application of the Ritz combination principle. The transitions belonging to the inverted P branch were assigned after the first fit was carried out and the rotationql Fonstants were precise enough to predict these transitions. However, we were unable to assign’all measured lines. Some rather weak lines were observed, which may be c-type tra&ions of higher K, subbands, transitions from molecules in vibrationally excited states, or from other

ROTATION-INVERSION

41

SPECTRUM OF HDNNC b 3

1.5

;

5 2

I.0

-d L

P

.5

; ;;

0. ” -.5

-.a

-.6

-.4

170134.5

-.2 shlIt

i

.2

.4

.6

-.6

/Hfz

170135.0 frqquency /mz

170135.5

-.4

-.2

170071.0 d

0 shift

.2

.4

.6

.B

A&

170071.5 frqumcy &Hz

170072.0

~i~~~~I~~~~~._‘~i~~~~;~‘T_7

-1.0

-1.0 -1.5

-I

169975.5

-.5 rhlfl

0 Rib

169576 169976.5 frqumcy

I

.5

-1.5

-1

-.5

i shill

.5 /Mb

I

1.5

2

I..I...I..I..

169577 lm77.5 t-Hk

lsmll

FIG. 7. Typical quadrupole hyperfme structures of transitions of HJVNC. The upper traces were observed, while the lower traces were calculated using a least-squares fitting program. The numbering (a) to (d) of the figures corresponds to the lineshape types quoted in Table V: Fig.

Transition

Unperturbed frequency (MHz)

a b

8 (4, 5,0-)- 7 (4, 4,0_) 8 (5, 4,0+)- 7 (5, 3,0+)

170 135.0316 170 071.5125

:

288 (6, (1,28,0+)-28 3,0-)- 7 (6, (0,28,0-) 2,0-)

216 169 976.7151 813.2329

isotopic species (H*NNC, DzNNC). For DJWC some strong a-type R-branch transitions were assigned, but the information they yielded was insufficient to determine precise rotational constants. (3) Influence of the Coriolis Resonance and Analysis of the Spectrum By employing our earlier method (8,23) of analysis with two separate Hamiltonians for the two inversion states O+ and O-, which are separated by the inversion splitting energy +JL, we were unable to fit and reproduce all measured transition frequencies for HDNNC. Some lines showed approximately equal deviations of opposite sign in the O+ and O- data sets. Inspection of the energy level diagram displayed in Fig. 4 and

42

WINNEWISSER AND REINSTAEDTLER TAR1 F IV

Quadrupole Coupling Constants Determined from Selected Transitions of HDNNC Transition vl

J(Ka.Kc,

-

Unperturbed

J(Ka.Kc.

frequency

VI

6( 3, 4,0+1

- 5( 3. 3.0+)

120

61 3. 4.0-j

- 5( 3. 3.0-j

120

880.9669

8( 5, 4,o+j

- 7( 5. 3,Of)

161

086.2461

8( 5, 4,O‘)

- 7( 5, 3.0-j

161

085.0937

91 6, 4,0+)

- 8( 6. 3.0+)

181

160.2672

9( 6, 4.0-j

- 8( 6. 3.0-I

181

159.0614

'.atNH)

Xbb(NH1

xbblNCl

Xaa(NCl

Typea

881.9303

8( 4, 5.o+j

- 7( 4. 4,o+j

161

135.1745

8( 4, 5.0-j

- 7( 4. 4.0-1

161

133.9509

9( 5, 5,o+j

- 8( 5. 4.0+)

161

219.7060

9( 5. 5.0-j

- 8( 5, 4.0-1

161

216.4048

5.02716)

0.31

1.9

-0.95

h

b

5.1911141

0.31

1.9

-0.95

A

b

5.149(331

0.31

1.9

-0.95

A

b

5.068(15)

0.3!

1.9

-0.95

6

b,c

4.945(l)

0.31

1.9

-0.95

8

b,e

-0.95

C

b

6( 4. 3.0+)

- 5( 4, 2,0+)

120 853.6136

6( 4, 3,0-I

- 5( 4, 2.0-j

120

852.6761

6.11

0.31

1.66(l)

5( 1, 5.0-1

- 5( 0. 5,0+1

187

161.7858

5.11

0.051(12)

1.64

-0.82

0

6( 1, 6.0-J

- 6t 0, 6,0+1

186

163.2984

5.11

0.171t84)

1.64

-0.82

D

7( 1, 7.0-1

- 71 0, 7,0+1

185

033.8765

5.11

0.311(O)

1.64

-0.82

D

81 1, 8,0-l

- 81 0, 8,0+)

183

723.9431

5.11

0.404114)

1.64

-0.82

D

91 1. 9,0-1

- 9( 0. 9.0+1

182

257.8224

5.11

O.OOlf24)

1.64

-0.82

D

16( 1.16.0-I

-16(

0.16.0+)

167

943.5303

5.11

0.36211)

1.64

-0.82

D

16( 1,16,0+1

-16(

0.16.0-1

160

956.2184

5.11

0.40316)

1.64

-0.82

D

17( 1,17,0-1

-171 0,17.0+)

165

373.8684

5.11

0.383(2)

1.64

-0.82

D

17( 1,17,0+1

-171 0,17,0‘)

158

390.3918

5.11

0.396(l)

1.64

-0.82

D

32(

1,32,0+)

-32(

109

907.4587

6.11

0.367(O)

1.64

-0.82

D

33(

1.33.0-J

-33(

0.33.0’l

113

251.0918

5.11

0.408(O)

1.64

-0.82

0

33(

1,33,0+x

-33(

0,33.0-l

106

347.9305

5.11

0.400(351

1.64

-0.82

D

529.9924

0,32.0-)

19(

0.19.0-1

-18(

1,17.0+1

169

19(

0.19,0+1

-18(

1,17.0-I

162 681.7328

5.11

0.390(6)

1.64

-0.82

D

5.11

0.400(4)

1.64

-0.82

0

221

0,22,0-1

-21(

1.20,0+)

218 351.2258

5.11

0.37918)

1.64

-0.82

D

22(

0,22.0+1

-21(

1,20.0-)

211 426.1326

5.11

0.393(45)

1.64

-0.82

D

231

0,23,0-j

-22(

1.21,0+)

234 132.6903

5.11

0.320(22)

1.64

-0.82

D

23(

0,23.0+)

-22(

1,21.0-)

227 215.9259

5.11

0.381llOl

1.64

-0.82

D

a)

The

typ(ca1

llnerhapes

observed

transition

b) Doublet.

COnsfstIng

quadrupole C) Meakly Table

patterns

split

Notes

are shown

in Fig.

61-d.

where

a, b, c, or d corresponds

e

to the type of the

proflle. The asymmetry

of the D+ and 0- transitions. were

lineshape.

fitted

doublet

was

unresolved.

Both

slmultaniausly.

4 weight

of 0.5 was

used

in the calculation

of the average

Constants

III

VI.

d) Ouadrupole calculating e) Perturbed

pattern

is perturbed

the average quadrupole

values

llneshape:

by the Corlolls in Table Coupling

resonance.

The coupling

VI. Constants

were

not used.

constants

were

not used

in

ROTATION-INVERSION

5

SPECTRUM OF HDNNC

6

7

a

43

9

J” FIG. 8. Change of the x66 (NH) quadrupole coupling constant as a function of J quantum number due to the decreasing influence of the Coriolis resonance.

the symmetry of the wavefunctions of the energy eigenvalues indicate the possibility of a Coriolis resonance interaction. After the inclusion of the a-type Coriolis resonance interaction terms in the Hamiltonian as discussed above, we were able to reproduce almost all the transition frequencies within the experimental accuracy. The observed and calculated transition frequencies are given in Table II. The influence of the Coriolis resonance on the transition frequencies was observable in the a-type R branch for K, = 1 lines with low J quantum numbers and for K, = 2 lines with high J quantum numbers. In the c-type @branch transitions at low J, the effects of the Coriolis resonance could also be observed (see following section). An example of the influence of the Coriolis resonance. on the rotational energy eigenvalues is sketched in Fig. 4. To demonstrate the effect of these Coriolis-induced shifts of the energy eigenvalues on the measured transition frequencies, Fig. 5 shows the shift in frequency relative to the unperturbed line positions of the a-type R-branch transitions (K, = 2,0+ system) versus the rotational quantum number J. The unusual frequency shifts can be understood by inspection of the energy level diagram and the energy shifts indicated there. Unfortunately, we were unable to observe all the shifted lines shown in Fig. 5; again the solid symbols represent the measured transitions. With the Hamiltonian given in Eqs. (6)-(g), we fitted the 173 measured and assigned transitions, using the least-squares method. The adjusted rotational constants are listed in Table III. We found that none of the centrifugal distortion constants differed significantly for the two inversion states O+ and O-. Therefore, they could be fitted for both states together in order to reduce the number of variable parameters, without any significant loss in accuracy. To allow a direct comparison with the rotational constants of HJVNC, we have included the average value of those constants in Table III. The measured transition frequencies and their deviations from the calculated values, together with some predicted line frequencies, are listed in Table II. The standard deviation of the best fit (Table III) was 29.3 kHz for 173 transitions which corresponds to the absolute frequency accuracy of the spectrometer. (4) Nuclear Quadrupole Hyperjine Structure in HDNNC Monodeuterated isocyanamide, HDNNC, possesses three nuclei which have nuclear quadrupole moments. These are the two nonequivalent nitrogen nuclei, indicated as

44

WINNEWISSER AND REINSTAEDTLER TABLE V Quadrupole Coupling Constants Determined from Selected Transitions of H,NNC Transition

J(Ka,Kc,

Unperturbed

V) - J(Ka,Kc,

VI

Frequency

Kaa("H'

Xbb(NH'

%a(%)

xbblNC)

Typea

Notes

8( 4, 5.0')

- 7( 4, 4,0+1

170

139.3605

5.156(15)

0.4

1.64

-0.82

A

b

B( 4, 5.0-j

- 7( 4, 4.0-i

170

135.0316

5.050(22)

0.4

1.64

-0.82

A

b

8( 5. 4.o+j

- 7( 5, 3,0+)

170

071.5125

5.092(120)

0.4

1.64

-0.82

B

b

5( 3. 3,0+1

- 4( 3, z.o+j

106

372.2150

5.393(62)

0.4

1.669113)

-0.9

C

b,c

8( 6, 3.0+1

- ?( 6. 2.0')

169

980.5033

5.153(17)

0.4

1.836121)

-0.9

c

b

8( 6. 3.0-j

- 7( 6. 2.0-j

169

976.7151

5.193(22)

0.4

1.818(33)

-0.82

C

b

5.14616)

0.4

1.876(6)

-0.9

C

b

5.113(25)

0.4

1.867(O)

-0.9

C

b

22(

1.22.0+1

-22(

0.22,0-1

232

664.4074

5.12

0.628(4)

1.6

-0.9

0

23(

1,23,0+)

-23(

0.23,0-)

230

224.6412

5.12

0.633(13)

1.8

-0.9

0

24(

1,24,0+)

-24(

0.24.0-I

227

700.3279

5.12

0.632(l)

1.8

-0.9

0

251

1.25.0')

-25(

0,25.0-j

225

094.1624

5.12

0.629(15)

1.8

-0.9

0

26(

1.26,0+)

-261

0,26,0-)

222

408.9819

5.12

0.652(S)

1.8

-0.9

0

27(

1.27,0+)

-2?[

0,27.0-)

219

647.6711

5.12

0.741(l)

1.8

-0.9

0

28(

1,2B.O+)

-28(

0.28.0-)

216

813.2329

5.12

0.638(2)

1.8

-0.9

D

291

1,29.0+)

-29(

0.29,0-)

213

908.7939

5.12

0.662143

1.8

-0.9

D

21( 0,21.0-)

-2O(

1.19.0+)

160

580.9721

5.12

0.619(31

1.8

-0.9

0

221

0.22.0+1

-21f

1.20.0-1

157

552.9493

5.12

0.612(25)

1.8

-0.9

0

24(

0.24.0-1

-23(

1.22.0+)

216

019.6268

5.12

0.643(11)

1.8

-0.9

cl

a) The

typical

observed bl The

lineshapes

transition

asymmetry

c) Perturbed average

doublet

quadrupole values

are

shown

in Fig.

7a-d,

where

a, b, c, or d corresponds

to the

type

c

of the

profile. was

unresolved.

pattern.

in Table

the coupling

constants

were

not used

in the calculation

of the

VI.

NH and NC, and the deuterium

nucleus. All three nuclei have nuclear spins of 1. Thus, a complex hyperfme pattern can be expected for each rotational transition, as has been observed and analyzed for example in diazirine (3). However, no transition observed showed more than four hyperfine features as can be seen from Fig. 6. Most of the transitions show only the 2: 1 doublet splitting mentioned earlier and displayed

ROTATION-INVERSION

SPECTRUM OF HDNNC

45

TABLE VI Mean Values of Nuclear Quadrupole Coupling Constants (in MHz) of HDNNC and H&WC HDNNC

xaaINH' XbbiNH)

Xcc'NH' ’

Xbb - xcc)(NH) xaa(%'

’

H*NNC

present work

Schafer (8,23)

5.093(84)a

5.129144)

5.300(500)b

0.391(14)

0.635114)

present work

-5.404

-5.764

5.875

6.399

1.65 c

1.849(23)

-6.ooof6oo~b 0.7

-6.7

1.8

xbbCNC)

-0.83 d

-0.92 d

0.0

Xcc(Nc)

-0.83 d

-0.92 d

-1.8

xbb - Xcc)(NC)

0.0

0.0

1.8

Numbers fn parentheses are the standard deviations of the average values in units of the last digits. Estimated standard deviation, see Schafer (23). Only one transition was evaluated. There was no additional splitting observable in the c-type transitions, therefore (xbb - Xc,) = 0 for the NC-nucleus.

in Fig. 6d. As in HzNNC (see Fig. 7) only the 14N-nuclear quadrupole interactions are observed, the influence of the deuterium quadrupole could not be resolved. In the analysis of the quadrupole splitting a whole-line fitting program was used which adjusts the quadrupole coupling constants of the two nuclei, the linewidth, and the position of the unsplit line frequency to reproduce the lineshape recorded with the spectrometer. We analyzed the quadrupole hyperhne structures of 30 transitions. The individual results are shown in Table IV. As is generally the case for a near-prolate top molecule at moderate or high J, the a-type transitions were influenced mainly by the X~ coupling constants, while the splittings of the c-type transitions were determined by the (xbb - xcc) values. Usually a cyanide nitrogen exhibits a much stronger quadrupole coupling than the isocyanide nitrogen (29,X)). Thus, we expected small values for the quadrupole coupling constants of the isonitrile nitrogen Nc, while the amide nitrogen NH should show a stronger quadrupole coupling. The xa of the Nc was determined from a line profile analysis of the transition shown in Fig. 6c. Since the c-type transitions observed showed no additional splitting due to the Ne atom, we deduced that the difference (xbb - xcc) is almost zero for Nc. From this assumption we calculated the coupling constants xbb and xcc listed. Since a small difference (x~ - xcc) would only afkct the f&ted linewidth of the c-type transitions, the possible uncertainties of these two constants are consid-

46

WINNEWISSER

AND

REINSTAEDTLER

TABLE VII Remeasured Transition Frequencies of HJVNC Transition

Frequencies

/MHz

Difference

valuea

(new

/MHZ

J(K,.K,, v) - J(K,.K,, v)

new value

old

- old)

5( 3, 2,0+1 - 4( 3, 1,0+1

106 372.2150

106 372.266

-0.0510

8( 4, 4,0+) - 7( 4. 3,o+j

170 139.3605

170 139.3757 b

-0.0152 b

81 4, 4,0-) - 71 4, 3,O'1

170 135.0316

170 135.0328 b

-0.0012 b

8( 5, 3,0+) - 7( 5, 2,o+j

170 071.5125

170 071.5353 b

-0.0228 b

8( 6, 2,0+) - 7( 6, l,O+)

169 980.5033

169 980.4945 b

0.0088 b

8( 6, 2.0-j - 7( 6, 1,0-l

169 976.7151

169 976.7068 b

0.0083 b

22( 1,22,0+1 -22L 0,22.0-1

232 664.4074

232 664.404

0.0034

23( 1,23,0+) -23( 0,23,0-j

230 224.6412

230 224.646

-0.0048

24( 1,24,0+) -24( 0,24,0-)

227 700.3279

227 700.350

-0.0221

25( 1,25,0+) -251 0,25.0-)

225 094.1624

225 094.1573 b

0.0051 b

26( 1,26,0+) -26( 0,26,0-j

222 408.9819

222 408.9654 b

0.0165 b

271 1,27,0+) -27( 0,27,0-j

219 647.6711

219 647.6537 b

28( 1,28,0+) -28( 0,28,0-j

216 813.2329

216 813.234

-0.0011

29( 1,29,0+1 -29( 0,29,0-j

213 908.7939

213 908.775

0.0189

0.0174 b

211 0,21,0-) -2Of 1.19,0+)

160 580.9721

160 581.0209 b

-0.0488 b

22( 0,22,o+j -21( 1,20,0-j

157 552.9493

157 552.9317 b

0.0176 b

24( 0,24,0-j -231 1,22.0+)

216 019.6268

216 019.627

a)

Obtained

b)

Calculated

by

SchPfer from

the

et

al.

constants

-0.0002

(8,231. given

by

Schafer

et

al.

(8,231.

erably larger than for xaa (Nc). The quadrupole coupling constants of the Nu atom were determined from several transitions showing the 2: 1 doublet splittings. The typical lineshapes are shown in Fig. 6. The errors attached to the constants listed in Table VI are the standard deviations of the average values from all analyzed transitions listed in Table IV. By analyzing the quadrupole structure of some of the low J c-type Q-branch transitions, the influence of the Coriolis resonance on the quadrupole hyperhne structure of these lines could be observed. Figure 8 shows the change of xbb(NH) with decreasing J value, or increasing strength of the Coriolis resonance. This effect is an additional aspect of the Coriolis resonance described above. (5) Nuclear Quadrupole Hyperfine Structure in HzNNC After completing the quadrupole hyperfine structure analysis of HDNNC, we found a discrepancy between the coupling constants obtained for HDNNC and those reported

ROTATION-INVERSION

SPECTRUM

47

OF HDNNC

TABLE VIII Barrier to Planarity and Inversion Splitting between 0’ and O- States of Some Amines (in cm-‘) Inversion spllttlnq

Barrier

MOleCUl.2

parent

2023a

Ammonia

0.7943=

monodeutero

dideutero

n.4os93b

a.17o74b

Cyanamide

51oc

49.568d

33e

15e

Anlllne

526f

40.Bf

24.4f

13.4f

Isocyanamlde

20704

0.368h

al Papousek and Splrko I.261 c) Brown

et al.

bl Cohen and Picket (311 d) Read

113)

0.015j

0.11751

et al.

(15)

e) Tyler et al. 132)

fl Kydd and Krueger (331

g) Vincent and Dykstra (18)

hl Scha'fer et

i) present work

jl Jensen and Wu'new~sser

al.

(8,23) (9)

for the normal species (8, 23) as given in Table VI. Therefore a reanalysis was mandatory. We measured 17 selected transitions of the normal species, HzNNC, and reanalyzed the hyperhne structure. The results of the individual analyses are listed in Table V. We were able to confirm the xna constants reported earlier within their quoted uncertainties, but the xbb and xcc constants reported by Schafer et al. (8, 23) were in error. We found that a sign error had probably occurred in the calculation of xbb and xcc from the difference (Xbb - xec). In order to illustrate this, we have included in Table VI not only the newly found coupling constants, but also the old values given by Schafer et al. (8,23). As can be seen from Table VI, the nuclear quadrupole coupling constants for the two isotopic species of isocyanamide agree very well. In Table VII, we compare the transition frequencies of H2NNC recently measured, with those given by Schafer et al. (8, 23). v.

DISCUSSION

From the results collected above, we have shown that the rotation-inversion spectrum of monodeuterated isocyanamide presents distinct aspects different from that of the parent species. A set of observable Coriolis resonances has been identified and included in the analysis. A further resonance, which probably affects the c-type Qbranch transitions at high J, could not yet be analyzed. From the observed shifts of the rotational constants upon deuterium substitution, we conclude that the ab initio structure calculated by Vincent and Dykstra (18) represents the molecular structure very well. For a complete experimentally determined structure more isotopomers have to be measured and analyzed. The quadrupole hyperline structure of both isotopic species was analyzed extensively. This was made possible through the improved software. We can now contribute the HDNNC entries to Table VIII where the inversion splittings of various molecules are compared. The change of the inversion splitting upon deuteration is much larger, relative to the value in the H2 species, than in the other listed amines.

48

WINNBWISSER AND REINSTAEDTLER ACKNOWLEDGMENTS

The authors expresssincerethanks to Dr. Brenda P. Winnewisserfor many helpful discussionsand criticallyreadingand commentingon the manuscript.We alsothankDr. J. Koput for hishelpand comments withthe Coriolisresonancediscussion. The chemical preparation of HDNNC was carried out by F. Holland.

We thank him for his help. The experimental work was in part supported by funds from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. All calculations were carried out at the Hochschulrechenzentrum of the JustucLiebig-University, a service which is gratefully acknowledged. RECEIVED:

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