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Rotational spectroscopy of 15N-hydrogen cyanide dimer: Detection, relative stability and D-nuclear quadrupole coupling of deuterated species
ROTATIONAL
18 November 1983
CHEMICAL PHYSICS LETTERS
Volume 102. number 2.3
SPECTROSCOPY
OF ISN-HYDROGEN
CYANIDE DIMER:
DETECTION, RELATIVE STABILITY AND D-NUCLEAR QUADRUPOLE OF DEUTERATED
COUPLING
SPECIES
AJ. FILLERY-TRAVIS,
A.C. LEGON, L.C. WILLOUGHBY
Chktopl~cr hgold Laborarotics. Departnrmt of CItemistr_s, LJtdlersity College London. 20 Gordon Street. London WCIH OAJ. UK and
A.D. BUCKINGHAXI tim3wsit_v C2emicai Laborator_s. Lemfield Road. Cambridge CBZ IEW. UK Received
S Srptcmbrr
1963
The three pcwblc deuterJtcd species of the lsN-h> drown cl snide dimer hsve been identified by their rotational specr1.1 .md rhe following spectroscopic constants have been de:ermined for the vibrationalground states: For D(J’5N...llC’5N: Bo = 1605.6946(l) MHz. DJ = 1.68-1(3) kHz. xD( 1) = 162.9(14) LHz; for HC’“N...DC’5N: f3o = 1683.3736(l) MHz, DJ = 1 SSS(SJ Lllz. xD(?) = 175.6(16) hHz: .md for DC’5N...DC’sN: B. = 1604.4947(6) MHz. DJ = 1.64(l) kHz, xD(l) + xD(2) = 36S(_S) kHz. It is concluded that the t\\o monodeuterated species differ in zero-point energy by only a few cm-‘_
I_ Introduction We hwe used the high sensitivity of pulsed-norzle, Fourier-transiorm microwave spectroscopy to identify the three possible deuterated species of the 15N-hydrogen cyanide dimer (HC15N...DC15N, DC15N...HC15N and DC1sN_._DC’5N) by means of their ground-state rotational spectra. The high resolution of the technique has allowed accurate values of the D-nuclear quadrupole coupling constants to be determined in addition to rotational and centrifugal distortion constants. The esperiments reported here have two aims. First, we seek to establish whether. as is the case for water dimers [ I] and hydrogen fluoride dimers [3] when investigated at very low temperatures. only those isomers in mixed H/D species rhat involve a deuterium bond can be observed. Buckingham and Liu [3] have demonstrated. by considering hydrogen-bonded systems as if they were linear triatomic molecules A-H...B with three significant vibrational modes (the A-H stretching mode and the doubly degenerate hydrogen-
126
bond bending modes), that the differences in stabilities of H- and D-bonded species can be related to differences in the zero-point energies of these vibrations. Thus they predict that. while the species HF...DF and H20_..DOH are favoured over DF...HFand H?O...HOD by 163 and 165 cm-l respectively, the corresponding isomers HCN...DCN and DCN_..HCN differ in zeropoint energy by only -5 cm-l. The last result suggests that it might be possible to observe both singly deuterated isomers of (HCN), in a pulsed-nozz!e experiment, even though the effective temperature is only ~5 K. Our second aim is to determine the D-nuclear quadrupole coupling constants and to interpret these in terms of any electrical changes that accompany dimer formation. In order to isolate the effects of such coupling we have restricted our investigation to the 15N-containing species, thereby avoiding the complexities of multiple nuclear quadrupole coupling and making the analysis tractable_ Previous reports [4,5] of the rotational spectrum of (HCN), did not examine deuterated species under high resolution.
0 009-2614/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 102, number 2,3
CHEMICAL PHYSICS LEl’TERS
iS November 1983
2. Experimental The pulsed-nozzle, FT microwave spectrometer used in the experiments described here is of the type developed by Balle and Flygare [6] and has been briefly discussed elsewhere [7] _In order to observe J = 1 + 0 transitions of the various deuterated species of the r*N-hydrogen cyanide dimer the frequency range of the spectrometer has been extended at the lower end to 3 GHZ. Mixtures of DCiSN and HC15N dilute in argon that were pulsed from the nozzle were prepared by allowing HC15N at a pressure <30 Torr into the sample reservoir which had previously been flushed with D20 vapour and evacuated_ Argon was then added to the reservoir to give a total pressure xl atm. HCrSN was prepared by dropping H3P0, onto solid KC15N in vacua.
3. Results 3_1_ Spectroscopic constants
i
too
200 300 Frequency Offset I kHz
0
4 0
Fig. l_ D-nuclear quadrupole triplet in the J = 1 - 0 transition of DC15N...HC15N. Frequenciesare offset at a rateof 3.90625 kHz per point from 3211.5150 MHz and the spectrum was obtained by Fourier transformation of a time domain signal digitized at the rate of 0.5 ps per point for 512 points. The stick diagram indicates the positions and intensities of the calculated components.
Approximate rotational transition frequencies for each of the three deuterated species of (HC15N), were predicted using the geometry determined for the hydrogen cyanide dimer in the earlier pulsed-nozzle FT investigation [S] _As expected, because of zero-point effects transitions were observed at frequencies higher than those predicted. The lower J + 1 + J transitions of each of the three D-species were assigned unambiguously on the basis of their D-nuclear quadrupole hyperfine patterns and therefore we were able to distinguish the species DC15N_..HC15N fromDC15N...DC1SN even though both have very similar Bu values because the atom involved in the weak binding lies close to the centre of mass. By restricting the investigation to species based on 15N, we ensured that the observed hyperfme structure arose only from the D-nuclei (I = 1). Nevertheless, this could be completely resolved only for the J = 1 + 0 transition of each species. Fig. 1 illustrates the characteristic D-nuclear quadrupole triplet pattern in the J = 1 f 0 transition of DC15N...HC1SN while the more complicated multiplet exhibited by the same transition of DC15N...DCiSN is shown in fig. 2. In both cases the stick diagram indicates the pattern predicted from the spectroscopic
I
0
100
200
300
4
Frequency Offset / kHz Fis_ 2. D-nuclear quadrupole components in the .I = I+ 0 transition of DC?N...DC?N. Frequencies are offset from 3208.7753 MHz at a rate of 3.90625 kHz per point but otherwise the caption to fig. 1 applies. Note that a two-point Doppler splitting is observed in each component.
127
Volume
CHEMICAL
102, number 2,3
Table 1 Observed and calculated frequencies of hyperfine components Transition
18 November 1983
PHYSICS LETTERS
in rotational transitions of DC*5N...HC15N
DC1sN...HC15N
HCtSN..DCr5N
J’-J”
F’ c F”
l-0
l-l 2-I 0-l
32 11.4268 3211.3734 3211.2911
-1.3 0.1 0.1
3366.7828 3366.7326 3366.6506
-0.9 1.6 -1.1
2-l
1-I 2-l 3-2 7-2 i-o
6422.6325
-0.6
6733.3466
0.2
3-2
2-2 4-3 3-2 2-l
1 I
V,bs(>!HZ)
6422.7213 6422.7661
“ohs-neate(kHr)
9633.9847 9633.9969
4-3
C)
12845.1259
j-4
Cl
16056.1037
“ob&fHr)
a) { -(jr-;
nabs-“ea@Hr)
6733.4312a)
b)
6733.4683
1
10099.9600
0.7
10100.0362
{ 8:: 2.0
{ 1::;
10100.0497
0.1 -0.4
given below. We note that the Doppler dou[S] is uuresolved in fig. 1 and barely so in fig. 2. thus allowing a direct correspondence between the observed p&tern and the stick didgram. Observed frequencies of D-nuclear quadrupole components in transitions of the ntonodeuterated species are recorded in table 1. For the J = 5 +- 4 and 4 +- 3 transitions, only a single component was observed, in agreement with calculations which show that a large fraction of the transition intensity falls within a single linewidth and within 1 kHz of the unperturbed transition frequency vo_ Table 2 gives observed frequencies for DC’sN...DCISN. For ali transitions other than the J = 1 - 0 we record only a value of vo_ In these cases we observed a predominant, strong central line with weaker side components. IModel calculations confin that the strong central component falls within a few kHz of v. escept for the J = 2 * 1 transition_ The frequencies displayed in tables 1 and 2 were analysed by the following method to give rotational constants B,, centrifugal distortion constants DJ, and D-nuclear quadrupole coupling constants x~(JI), where II = 1 and 2 indicate that the D-nuclei are respectively uninvolved and involved in the intermolecular binding. biatrix elements of the hamiltonian bling phenotuenon
1%
(17-j b)
2.6
13466.5065
-0.1
16832.7938
-0.2
a) Observed frequency assumed to be the mean of the indicated components weighted according b) Not included in least-squares fit. c) All D-nuclear quadrupole components fall within the liwxidth and 11ithin 1 kiiz of “0.
constants
and HCr5N...DC*SN
H=HR
+HQ(l)+HQ(?)
were evaluated Z&)
to their calculated
+-Z,(z)
in the coupled =Z,
Z+J=F,
intensities.
(1)
basis (2)
using expressions given by Keenan et al. [9]. In eq. (l), the approximate eigenvalues of HR are BoJ(J + 1) - DJJ2( J + I)?, which is the usual expression for the rotational energy of a semi-rigid linear molecule, while H&r) is the well-known nuclear quadrupole interaction term for nucleus JZ [lo]. Elements of HQ(Jz) offdiagonal in J were ignored owing to the inequality HR % H&Z) (typically, ER z 10 GHz while Ea z 100 kHz). Such a first-order treatment is entirely satisfactory in view of the accuracy (1 kHz) of frequency measurement. For each of the monodeuterated species, HQ(2) and ZD(2) were set to zero, thus generating the diagonal matrix of HQ in the Z + J = F basis that is familiar in first-order treatments of quadrupole coupling involving a single nucleus [lo] _Then observed hyperfine frequencies were fitted in a least-squares analysis to give Bo, DJ and the appropriate xD(?z), with results given in table 3. Differences between observed and calculated frequencies are in table l_
CHEMICAL
Volume 102; number 2,3
Table 2 Observed and calculated frequencies of hyperfme components Transition J’c
18 November
PHYSICS LETTERS
in rotational
1983
transitions of DC’5N__.DC1SN
uobs(MHa)
nabs - ncaIc(kHr)
I'F'.-1°F
J"
21 11 23 12 01 01
I+0
+22 cl1 +22 +I1 +22 coo -22 +ll
22 10 J+l+J 1 2 3 4 5
vu values:
3208.8663 3208.9384 3208.9671 3208.9943 1
-0 +O -2 +3 +4
1.4 -1.2 -0.1 -0.5
3209.0398
-2.0
3209.0476 3209.0818
-2.4 4.1
3208.9856(10) 6417.9263 9626.7888C) 12835.539OC) 16044.1266C)
2.7 a) -b) -2.3 1.3 -0.2
a) Difference of observed and calculated frequencies in the fit of ne values to Bo and DJ_ b) IJOestimated from simulation of partially resolved hyperfme structure using xD(n) values and not included in the analysis for B,J and DJ. C) Nuclear quadrupole hypertine structure dominated by a strong, central transition close to the unperturbed frequency no_ Quoted value is the estimated value of wo. Table 3 Spectroscopic
constants of DC’5N...HC15N.
HC’SN...DCr5N
and DC15N...DC’5N
Spectroscopic constant
DC’5N...HC’5N
HC15N...DC1SN
Dt?N
B. (MHz)
1605.6946(l) l-684(4) 182.9(14) -
1683_3736(2) 1X85(5) -
1604_4947(6) 1.64(l)
DJ (kHr) XD( 1) &Hz) xD(2) &Hz) xD(I) l xD(2)
(kHz)
a) Determined from the J = 1~
0 transition by assuming xD(1)
For DC15N..DC15N the hyperfine structure was analysed only for the J = 1 + 0 transition. The frequencies of components in this transition depend strongly on ~~(1) + ~~(2) but only weakly on the small quantity ~~(1) - ~~(2). Accordingly, in the least-squares analysis ~~(1) - ~~(2) was held fued at the value obtained from the monodeuterated dimers while v. and xD(l)+xD(2) were fitted to the observed frequencies. In a second step, this v. value and all of the others from table 2 were fitted to give B. and DP The spectroscopic constants of the dideuterated molecule are included in table 3 while the residuals for the frt of the J = 1 +- 0 transition are given in table 2.
175.6(16) -
.__DCr5N
368(5)
a)
- xD(2) = 7.3 kHz (see text for discussion)_
3.2. Relative stability HCIsN . ..DC”N
of DC1sN_..HC15N
and
The nature of the observed spectra and the spectroscopic constants presented in table 3 show unarn-
biguously that both of the species DC15N...HC15N and HC15N..DC15N have been detected through their rotational spectra by using the pulsed-nozzle, FT microwave spectrometer. Moreover, we observed no detectable difference in spectral intensity for these isomers and consequently, since the effective temperature of the dimers emerging from the nozzle is expected to be 6 K, the difference in zero-point energy of the 129
CHEMICAL
Volume 102, number 2,3 two monodeuterated
isomers
is at most
only
PHYSICS LETTERS small
a few
cm-l. This result is in agreement with the prediction [3] that the H-bonded isomer is only slightly more stable than the D-bonded species. The relative stability of the D- and H-bonded isomers of the hydrogen cyanide dimer presents a marked contrast with that in the hydrogen fluoride dimer and the water
dimer.
be detected
Thus,
only
the HF...DF
species
could
in
HF/DF mixtures by molecular beam electric resonance spectroscopy [2]. while examination by the same technique of multiply deuterated isomers of the water dimer indicated that only those involving a deuterium bond were observed [ I] _In addition, we recently searched with the present technique for monodeuterated water dimers using predictions based on the model favoured in ref. [ 1] _The 1o1 -Ooo transition of H20...DOH was identified at 123 12.5 MHz by means of the characteristic D-nuclear quadrupole triplet shown in fig. 3 (at such frequencies the Doppler splitting is well resolved). The identification is confirmed by the fact that this transition occurs only 6 MHz higher than predicted and only 8 MHz lower than the same transition of (HzO)z_ In the latter case. only D-substitution that is close to the centre of mass could give such a
1 0
!
t
8
100 Frequency
I
200 Offset
1
300
4
/ kHz
Fig. 3. D-nuclear quadrupole components in the lol--Ooo transition of HzO...DOH. Frequencies are offset from 17312.7199 MHz at a rate of 3.90625 kHz per point. Each component exhibits an eight-point Doppler splitting.
130
18 November 1983
3
shift. Fresumably, any hyperfine splitting arising from the three proton spins in HzO...DOH is too small to resolve and under this assumption the triplet in fig. 3 can be fitted to give 2 = 240(2) kHz and v. = 123 12.5 197(7) MHz. The relatively large value of g (compared with 308 kHz along the 0-D bond in HOD [ 111) is consistent with assignment to the species H,O.._DOH, for the other isomer would have a very much smaller value and in any case its 1oi -Ooo transition is predicted to occur at 11811 MHz. A careful search in this region failed to reveal the dimer H20...HOD. Thus all results so far reported for hydrogen fluoride and water are consistent with the predicted stability [3] of =I65 cm-’ for the D-bonded over H-bonded isomers since at ~5 K the latter would not be populated_ 3.3. Interpretation of the D-nuclear quadnipole coupling constants The customary starting point for the interpretation of nuclear quadrupole coupling constants of weakly bound dimers is the expression
x%0
= + x;
(3 co&,
-
l)op ,
(3)
where $= 194.4(25) kHz is the appropriate value for free DCN [ 121 and 8,z is the angle between the DCN internuclear line and the instantaneous a axis of the dimer during the zero-point motion, over which the average is taken. Use of eq. (3) implies the electric field gradients at the D-nuclei survive dimer formation_ If eq. (3) is taken to define operationally an average value of 0,*, the xD(?z) values of table 3 lead to 8 1 = 11.5(2.1)” and 62 = 14.7(1.7)“. Such angles have been similarly determined from the 14N nuclear quadrupole coupling constants xN(tz) of HC14N...HC15N and HC15N...HC14N as 0 1 = 17.0° and 8, = 11.4” respectively. It is instructive to compare these Bn determined from X~(JZ) and xN(n)_ Following Buxton et al_ [S]) we assume that in the zero-point state the motion consists of the oscillation of each HCN subunit with respect to its own centre of mass. Thus in comparing 8, determined from HC14N...HC15N and DC15N...HC15N, we expect this quantity to decrease substantially as a result of the increased mass of the first subunit which reduces its oscillation. We note that a decrease from 17.0 to 11.5” is indeed observed_ On the other hand, the decrease
Volume 102, number 2,3
CHEMICAL-PHYSICS
of O2 likewise expected when passing from HC’SN_..HC14N to HClsN...DC15N does not occur end in fact 82 increases from 11.4” to 14.7O. We can rationalise the increase in 82 if we assume that in forming the D-bond in HC15N..DC15N there is a significant change in the electric field gradient at D along the subunit axis, which leads to a decrease of ~~(2) in addition to that arising from the zero-point motion in the dimer. Since the 14N-nuc1eus in HC15N.._HC14N is further from the other subunit, changes in the field gradient there are expected to contribute much less to ~~(2) and hence to the angle O2 so determined_ We note that angles operationally defined in a similar manner from xD values in several strongly bond species B...DF are likewise larger than those appropriate to the corresponding B...HF species and determined from H, lgF nuclear spin-nuclear spin coupling constants [3] _
Acknowledgement
LETTERS
that allowed us to extend the lower-frequency limit of the spectrometer to 3 GHz.
[l] [ 21 [3] [4] [5] [6] [7]
[ 81 [ 91 [lo]
The support of the SERC through an equipment grant and a research studentship (AJFT) is gratefully acknowledged_ We thank Professor J.R. Forrest and CJ. Ward of the Department of Electrical and Electronic Engineering at UCL for the loan of oscillators
18 November 1983
[ 111 [ 121
[ 131
J.A. Odutola and T-R. Dyke, 1. Chem. Phys. 72 (1980) 5602. T-R. Dyke, B-J. Howard and W. Klemperer, J. Chem. Phys. 56 (1972) 2442. A-D. Buckingham and Liu FanChen, Intern. Rev. Phys. Chem. 1 (1981) 253. AC. Legon, D.J. hlillen and P.J. Miobcrg, Chem. Phys. Letters 47 (1977) 589. L.W. Buxton. EJ. Campbell and W-H. Flygate. Chem. Phys. 56 (1981) 399. T-J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. AC. Legon and L.C. Willoughby, Chem. Phys. 74 (1983) 127. E-J. Campbell, L.W. Buxton,T.J. Balle, M.R. Keenan and W.H. Flygare, J. Chem. Phys. 74 (1981) 829. MR. Keenan. D.B. Wozniak and W.H. Flygare, J. Chem. Phys. 75 (1981) 631. C.H. Townes and A.L. Schawlow, Microwave spectroscopy (hlcGraw-Hill. New York, 1955). J. Verhoeven. A. Dymanus and H. Bluyssen. J. Chem. Phys. 50 (1969) 3330. F-C. DeLucia and W. Gordy, Phys. Rev. 187 (1969) 58. AC. Legon and L.C. Willoughby, Chem. Phys. Letters 92 (1982) 333.
131
18 November 1983
CHEMICAL PHYSICS LETTERS
Volume 102. number 2.3
SPECTROSCOPY
OF ISN-HYDROGEN
CYANIDE DIMER:
DETECTION, RELATIVE STABILITY AND D-NUCLEAR QUADRUPOLE OF DEUTERATED
COUPLING
SPECIES
AJ. FILLERY-TRAVIS,
A.C. LEGON, L.C. WILLOUGHBY
Chktopl~cr hgold Laborarotics. Departnrmt of CItemistr_s, LJtdlersity College London. 20 Gordon Street. London WCIH OAJ. UK and
A.D. BUCKINGHAXI tim3wsit_v C2emicai Laborator_s. Lemfield Road. Cambridge CBZ IEW. UK Received
S Srptcmbrr
1963
The three pcwblc deuterJtcd species of the lsN-h> drown cl snide dimer hsve been identified by their rotational specr1.1 .md rhe following spectroscopic constants have been de:ermined for the vibrationalground states: For D(J’5N...llC’5N: Bo = 1605.6946(l) MHz. DJ = 1.68-1(3) kHz. xD( 1) = 162.9(14) LHz; for HC’“N...DC’5N: f3o = 1683.3736(l) MHz, DJ = 1 SSS(SJ Lllz. xD(?) = 175.6(16) hHz: .md for DC’5N...DC’sN: B. = 1604.4947(6) MHz. DJ = 1.64(l) kHz, xD(l) + xD(2) = 36S(_S) kHz. It is concluded that the t\\o monodeuterated species differ in zero-point energy by only a few cm-‘_
I_ Introduction We hwe used the high sensitivity of pulsed-norzle, Fourier-transiorm microwave spectroscopy to identify the three possible deuterated species of the 15N-hydrogen cyanide dimer (HC15N...DC15N, DC15N...HC15N and DC1sN_._DC’5N) by means of their ground-state rotational spectra. The high resolution of the technique has allowed accurate values of the D-nuclear quadrupole coupling constants to be determined in addition to rotational and centrifugal distortion constants. The esperiments reported here have two aims. First, we seek to establish whether. as is the case for water dimers [ I] and hydrogen fluoride dimers [3] when investigated at very low temperatures. only those isomers in mixed H/D species rhat involve a deuterium bond can be observed. Buckingham and Liu [3] have demonstrated. by considering hydrogen-bonded systems as if they were linear triatomic molecules A-H...B with three significant vibrational modes (the A-H stretching mode and the doubly degenerate hydrogen-
126
bond bending modes), that the differences in stabilities of H- and D-bonded species can be related to differences in the zero-point energies of these vibrations. Thus they predict that. while the species HF...DF and H20_..DOH are favoured over DF...HFand H?O...HOD by 163 and 165 cm-l respectively, the corresponding isomers HCN...DCN and DCN_..HCN differ in zeropoint energy by only -5 cm-l. The last result suggests that it might be possible to observe both singly deuterated isomers of (HCN), in a pulsed-nozz!e experiment, even though the effective temperature is only ~5 K. Our second aim is to determine the D-nuclear quadrupole coupling constants and to interpret these in terms of any electrical changes that accompany dimer formation. In order to isolate the effects of such coupling we have restricted our investigation to the 15N-containing species, thereby avoiding the complexities of multiple nuclear quadrupole coupling and making the analysis tractable_ Previous reports [4,5] of the rotational spectrum of (HCN), did not examine deuterated species under high resolution.
0 009-2614/S3/0000-0000/S
03.00 0 1983 North-Holland
Volume 102, number 2,3
CHEMICAL PHYSICS LEl’TERS
iS November 1983
2. Experimental The pulsed-nozzle, FT microwave spectrometer used in the experiments described here is of the type developed by Balle and Flygare [6] and has been briefly discussed elsewhere [7] _In order to observe J = 1 + 0 transitions of the various deuterated species of the r*N-hydrogen cyanide dimer the frequency range of the spectrometer has been extended at the lower end to 3 GHZ. Mixtures of DCiSN and HC15N dilute in argon that were pulsed from the nozzle were prepared by allowing HC15N at a pressure <30 Torr into the sample reservoir which had previously been flushed with D20 vapour and evacuated_ Argon was then added to the reservoir to give a total pressure xl atm. HCrSN was prepared by dropping H3P0, onto solid KC15N in vacua.
3. Results 3_1_ Spectroscopic constants
i
too
200 300 Frequency Offset I kHz
0
4 0
Fig. l_ D-nuclear quadrupole triplet in the J = 1 - 0 transition of DC15N...HC15N. Frequenciesare offset at a rateof 3.90625 kHz per point from 3211.5150 MHz and the spectrum was obtained by Fourier transformation of a time domain signal digitized at the rate of 0.5 ps per point for 512 points. The stick diagram indicates the positions and intensities of the calculated components.
Approximate rotational transition frequencies for each of the three deuterated species of (HC15N), were predicted using the geometry determined for the hydrogen cyanide dimer in the earlier pulsed-nozzle FT investigation [S] _As expected, because of zero-point effects transitions were observed at frequencies higher than those predicted. The lower J + 1 + J transitions of each of the three D-species were assigned unambiguously on the basis of their D-nuclear quadrupole hyperfine patterns and therefore we were able to distinguish the species DC15N_..HC15N fromDC15N...DC1SN even though both have very similar Bu values because the atom involved in the weak binding lies close to the centre of mass. By restricting the investigation to species based on 15N, we ensured that the observed hyperfme structure arose only from the D-nuclei (I = 1). Nevertheless, this could be completely resolved only for the J = 1 + 0 transition of each species. Fig. 1 illustrates the characteristic D-nuclear quadrupole triplet pattern in the J = 1 f 0 transition of DC15N...HC1SN while the more complicated multiplet exhibited by the same transition of DC15N...DCiSN is shown in fig. 2. In both cases the stick diagram indicates the pattern predicted from the spectroscopic
I
0
100
200
300
4
Frequency Offset / kHz Fis_ 2. D-nuclear quadrupole components in the .I = I+ 0 transition of DC?N...DC?N. Frequencies are offset from 3208.7753 MHz at a rate of 3.90625 kHz per point but otherwise the caption to fig. 1 applies. Note that a two-point Doppler splitting is observed in each component.
127
Volume
CHEMICAL
102, number 2,3
Table 1 Observed and calculated frequencies of hyperfine components Transition
18 November 1983
PHYSICS LETTERS
in rotational transitions of DC*5N...HC15N
DC1sN...HC15N
HCtSN..DCr5N
J’-J”
F’ c F”
l-0
l-l 2-I 0-l
32 11.4268 3211.3734 3211.2911
-1.3 0.1 0.1
3366.7828 3366.7326 3366.6506
-0.9 1.6 -1.1
2-l
1-I 2-l 3-2 7-2 i-o
6422.6325
-0.6
6733.3466
0.2
3-2
2-2 4-3 3-2 2-l
1 I
V,bs(>!HZ)
6422.7213 6422.7661
“ohs-neate(kHr)
9633.9847 9633.9969
4-3
C)
12845.1259
j-4
Cl
16056.1037
“ob&fHr)
a) { -(jr-;
nabs-“ea@Hr)
6733.4312a)
b)
6733.4683
1
10099.9600
0.7
10100.0362
{ 8:: 2.0
{ 1::;
10100.0497
0.1 -0.4
given below. We note that the Doppler dou[S] is uuresolved in fig. 1 and barely so in fig. 2. thus allowing a direct correspondence between the observed p&tern and the stick didgram. Observed frequencies of D-nuclear quadrupole components in transitions of the ntonodeuterated species are recorded in table 1. For the J = 5 +- 4 and 4 +- 3 transitions, only a single component was observed, in agreement with calculations which show that a large fraction of the transition intensity falls within a single linewidth and within 1 kHz of the unperturbed transition frequency vo_ Table 2 gives observed frequencies for DC’sN...DCISN. For ali transitions other than the J = 1 - 0 we record only a value of vo_ In these cases we observed a predominant, strong central line with weaker side components. IModel calculations confin that the strong central component falls within a few kHz of v. escept for the J = 2 * 1 transition_ The frequencies displayed in tables 1 and 2 were analysed by the following method to give rotational constants B,, centrifugal distortion constants DJ, and D-nuclear quadrupole coupling constants x~(JI), where II = 1 and 2 indicate that the D-nuclei are respectively uninvolved and involved in the intermolecular binding. biatrix elements of the hamiltonian bling phenotuenon
1%
(17-j b)
2.6
13466.5065
-0.1
16832.7938
-0.2
a) Observed frequency assumed to be the mean of the indicated components weighted according b) Not included in least-squares fit. c) All D-nuclear quadrupole components fall within the liwxidth and 11ithin 1 kiiz of “0.
constants
and HCr5N...DC*SN
H=HR
+HQ(l)+HQ(?)
were evaluated Z&)
to their calculated
+-Z,(z)
in the coupled =Z,
Z+J=F,
intensities.
(1)
basis (2)
using expressions given by Keenan et al. [9]. In eq. (l), the approximate eigenvalues of HR are BoJ(J + 1) - DJJ2( J + I)?, which is the usual expression for the rotational energy of a semi-rigid linear molecule, while H&r) is the well-known nuclear quadrupole interaction term for nucleus JZ [lo]. Elements of HQ(Jz) offdiagonal in J were ignored owing to the inequality HR % H&Z) (typically, ER z 10 GHz while Ea z 100 kHz). Such a first-order treatment is entirely satisfactory in view of the accuracy (1 kHz) of frequency measurement. For each of the monodeuterated species, HQ(2) and ZD(2) were set to zero, thus generating the diagonal matrix of HQ in the Z + J = F basis that is familiar in first-order treatments of quadrupole coupling involving a single nucleus [lo] _Then observed hyperfine frequencies were fitted in a least-squares analysis to give Bo, DJ and the appropriate xD(?z), with results given in table 3. Differences between observed and calculated frequencies are in table l_
CHEMICAL
Volume 102; number 2,3
Table 2 Observed and calculated frequencies of hyperfme components Transition J’c
18 November
PHYSICS LETTERS
in rotational
1983
transitions of DC’5N__.DC1SN
uobs(MHa)
nabs - ncaIc(kHr)
I'F'.-1°F
J"
21 11 23 12 01 01
I+0
+22 cl1 +22 +I1 +22 coo -22 +ll
22 10 J+l+J 1 2 3 4 5
vu values:
3208.8663 3208.9384 3208.9671 3208.9943 1
-0 +O -2 +3 +4
1.4 -1.2 -0.1 -0.5
3209.0398
-2.0
3209.0476 3209.0818
-2.4 4.1
3208.9856(10) 6417.9263 9626.7888C) 12835.539OC) 16044.1266C)
2.7 a) -b) -2.3 1.3 -0.2
a) Difference of observed and calculated frequencies in the fit of ne values to Bo and DJ_ b) IJOestimated from simulation of partially resolved hyperfme structure using xD(n) values and not included in the analysis for B,J and DJ. C) Nuclear quadrupole hypertine structure dominated by a strong, central transition close to the unperturbed frequency no_ Quoted value is the estimated value of wo. Table 3 Spectroscopic
constants of DC’5N...HC15N.
HC’SN...DCr5N
and DC15N...DC’5N
Spectroscopic constant
DC’5N...HC’5N
HC15N...DC1SN
Dt?N
B. (MHz)
1605.6946(l) l-684(4) 182.9(14) -
1683_3736(2) 1X85(5) -
1604_4947(6) 1.64(l)
DJ (kHr) XD( 1) &Hz) xD(2) &Hz) xD(I) l xD(2)
(kHz)
a) Determined from the J = 1~
0 transition by assuming xD(1)
For DC15N..DC15N the hyperfine structure was analysed only for the J = 1 + 0 transition. The frequencies of components in this transition depend strongly on ~~(1) + ~~(2) but only weakly on the small quantity ~~(1) - ~~(2). Accordingly, in the least-squares analysis ~~(1) - ~~(2) was held fued at the value obtained from the monodeuterated dimers while v. and xD(l)+xD(2) were fitted to the observed frequencies. In a second step, this v. value and all of the others from table 2 were fitted to give B. and DP The spectroscopic constants of the dideuterated molecule are included in table 3 while the residuals for the frt of the J = 1 +- 0 transition are given in table 2.
175.6(16) -
.__DCr5N
368(5)
a)
- xD(2) = 7.3 kHz (see text for discussion)_
3.2. Relative stability HCIsN . ..DC”N
of DC1sN_..HC15N
and
The nature of the observed spectra and the spectroscopic constants presented in table 3 show unarn-
biguously that both of the species DC15N...HC15N and HC15N..DC15N have been detected through their rotational spectra by using the pulsed-nozzle, FT microwave spectrometer. Moreover, we observed no detectable difference in spectral intensity for these isomers and consequently, since the effective temperature of the dimers emerging from the nozzle is expected to be 6 K, the difference in zero-point energy of the 129
CHEMICAL
Volume 102, number 2,3 two monodeuterated
isomers
is at most
only
PHYSICS LETTERS small
a few
cm-l. This result is in agreement with the prediction [3] that the H-bonded isomer is only slightly more stable than the D-bonded species. The relative stability of the D- and H-bonded isomers of the hydrogen cyanide dimer presents a marked contrast with that in the hydrogen fluoride dimer and the water
dimer.
be detected
Thus,
only
the HF...DF
species
could
in
HF/DF mixtures by molecular beam electric resonance spectroscopy [2]. while examination by the same technique of multiply deuterated isomers of the water dimer indicated that only those involving a deuterium bond were observed [ I] _In addition, we recently searched with the present technique for monodeuterated water dimers using predictions based on the model favoured in ref. [ 1] _The 1o1 -Ooo transition of H20...DOH was identified at 123 12.5 MHz by means of the characteristic D-nuclear quadrupole triplet shown in fig. 3 (at such frequencies the Doppler splitting is well resolved). The identification is confirmed by the fact that this transition occurs only 6 MHz higher than predicted and only 8 MHz lower than the same transition of (HzO)z_ In the latter case. only D-substitution that is close to the centre of mass could give such a
1 0
!
t
8
100 Frequency
I
200 Offset
1
300
4
/ kHz
Fig. 3. D-nuclear quadrupole components in the lol--Ooo transition of HzO...DOH. Frequencies are offset from 17312.7199 MHz at a rate of 3.90625 kHz per point. Each component exhibits an eight-point Doppler splitting.
130
18 November 1983
3
shift. Fresumably, any hyperfine splitting arising from the three proton spins in HzO...DOH is too small to resolve and under this assumption the triplet in fig. 3 can be fitted to give 2 = 240(2) kHz and v. = 123 12.5 197(7) MHz. The relatively large value of g (compared with 308 kHz along the 0-D bond in HOD [ 111) is consistent with assignment to the species H,O.._DOH, for the other isomer would have a very much smaller value and in any case its 1oi -Ooo transition is predicted to occur at 11811 MHz. A careful search in this region failed to reveal the dimer H20...HOD. Thus all results so far reported for hydrogen fluoride and water are consistent with the predicted stability [3] of =I65 cm-’ for the D-bonded over H-bonded isomers since at ~5 K the latter would not be populated_ 3.3. Interpretation of the D-nuclear quadnipole coupling constants The customary starting point for the interpretation of nuclear quadrupole coupling constants of weakly bound dimers is the expression
x%0
= + x;
(3 co&,
-
l)op ,
(3)
where $= 194.4(25) kHz is the appropriate value for free DCN [ 121 and 8,z is the angle between the DCN internuclear line and the instantaneous a axis of the dimer during the zero-point motion, over which the average is taken. Use of eq. (3) implies the electric field gradients at the D-nuclei survive dimer formation_ If eq. (3) is taken to define operationally an average value of 0,*, the xD(?z) values of table 3 lead to 8 1 = 11.5(2.1)” and 62 = 14.7(1.7)“. Such angles have been similarly determined from the 14N nuclear quadrupole coupling constants xN(tz) of HC14N...HC15N and HC15N...HC14N as 0 1 = 17.0° and 8, = 11.4” respectively. It is instructive to compare these Bn determined from X~(JZ) and xN(n)_ Following Buxton et al_ [S]) we assume that in the zero-point state the motion consists of the oscillation of each HCN subunit with respect to its own centre of mass. Thus in comparing 8, determined from HC14N...HC15N and DC15N...HC15N, we expect this quantity to decrease substantially as a result of the increased mass of the first subunit which reduces its oscillation. We note that a decrease from 17.0 to 11.5” is indeed observed_ On the other hand, the decrease
Volume 102, number 2,3
CHEMICAL-PHYSICS
of O2 likewise expected when passing from HC’SN_..HC14N to HClsN...DC15N does not occur end in fact 82 increases from 11.4” to 14.7O. We can rationalise the increase in 82 if we assume that in forming the D-bond in HC15N..DC15N there is a significant change in the electric field gradient at D along the subunit axis, which leads to a decrease of ~~(2) in addition to that arising from the zero-point motion in the dimer. Since the 14N-nuc1eus in HC15N.._HC14N is further from the other subunit, changes in the field gradient there are expected to contribute much less to ~~(2) and hence to the angle O2 so determined_ We note that angles operationally defined in a similar manner from xD values in several strongly bond species B...DF are likewise larger than those appropriate to the corresponding B...HF species and determined from H, lgF nuclear spin-nuclear spin coupling constants [3] _
Acknowledgement
LETTERS
that allowed us to extend the lower-frequency limit of the spectrometer to 3 GHz.
[l] [ 21 [3] [4] [5] [6] [7]
[ 81 [ 91 [lo]
The support of the SERC through an equipment grant and a research studentship (AJFT) is gratefully acknowledged_ We thank Professor J.R. Forrest and CJ. Ward of the Department of Electrical and Electronic Engineering at UCL for the loan of oscillators
18 November 1983
[ 111 [ 121
[ 131
J.A. Odutola and T-R. Dyke, 1. Chem. Phys. 72 (1980) 5602. T-R. Dyke, B-J. Howard and W. Klemperer, J. Chem. Phys. 56 (1972) 2442. A-D. Buckingham and Liu FanChen, Intern. Rev. Phys. Chem. 1 (1981) 253. AC. Legon, D.J. hlillen and P.J. Miobcrg, Chem. Phys. Letters 47 (1977) 589. L.W. Buxton. EJ. Campbell and W-H. Flygate. Chem. Phys. 56 (1981) 399. T-J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. AC. Legon and L.C. Willoughby, Chem. Phys. 74 (1983) 127. E-J. Campbell, L.W. Buxton,T.J. Balle, M.R. Keenan and W.H. Flygare, J. Chem. Phys. 74 (1981) 829. MR. Keenan. D.B. Wozniak and W.H. Flygare, J. Chem. Phys. 75 (1981) 631. C.H. Townes and A.L. Schawlow, Microwave spectroscopy (hlcGraw-Hill. New York, 1955). J. Verhoeven. A. Dymanus and H. Bluyssen. J. Chem. Phys. 50 (1969) 3330. F-C. DeLucia and W. Gordy, Phys. Rev. 187 (1969) 58. AC. Legon and L.C. Willoughby, Chem. Phys. Letters 92 (1982) 333.
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