J. Chem. Phys., Vol. 113, No. 13, 1 October 2000
Rotational spectroscopy of H P¯BrCl
5285
3
where r and rЈ are the distances of the P and Br atoms from
relatively small lengthening of the BrCl bond on formation
of the complex. By contrast, interpretations of the Br and Cl
nuclear quadrupole coupling constants, based on a simple
version of the Townes–Dailey model, reveal that not only is
the PH and BrCl centers of mass, respectively. Values of r
3
and rЈ appropriate to unperturbed PH isotopomers and BrCl
3
at its geometry in the complex are available from the geom-
etries included in Table IV.
there a significant intramolecular electronic transfer from Br
Cl
p
We note from Table V that r(P¯Br) is significantly
to Cl on complex formation ͓i.e., ␦ ϭ0.128(2)͔ but there is
shorter for D P¯BrCl isotopomers than for H P¯BrCl iso-
also a substantial intermolecular transfer. Indeed, it was
3
3
topomers. A similar effect has been observed in the com-
shown that a fraction ␦ ϭ0.10(1) of an electron moves from
i
1
1
31
plexes H P¯ICl and H P¯HCl. A possible source of the
P to Br.
3
3
3
2
effect is the increased electric dipole moment of PD rela-
One might expect that a large extent of electron transfer
would imply a stronger intermolecular bond but evidently
this is not the case for the pair of isostructural complexes
H P¯BrCl and H N¯BrCl. For example, application of the
3
tive to PH and therefore an increased interaction strength in
3
1
1
D P¯BrCl isotopomers, as discussed elsewhere. We also
3
Cl
p
note that both ␦ and ␦ ͑see Table V͒ appear larger in the
i
3
3
D P¯BrCl species, indicating an enhanced interaction.
analysis of the halogen nuclear quadrupole coupling con-
3
stants described in Sec. III B gives ␦ ϭ0.064(5) for
i
E. Strength of the intermolecular bond in H P¯BrCl
3
3
6
H N¯BrCl, but this complex is much more strongly
3
We can gauge the strength of the intermolecular binding
bound than H P¯BrCl, the k values being 26.7͑3͒ and
3
Ϫ1
in H P¯BrCl from the magnitude of the intermolecular
3
11.7͑1͒ N m , respectively. What is it that controls the mag-
nitude of ␦i?
stretching force constant k , which in the approximation that
terms higher than quadratic in the force field of the complex
are negligible gives the restoring force for unit infinitesimal
The extent of intermolecular electron transfer ␦ in com-
i
10
plexes B¯ICl has been shown to correlate with the appro-
displacement along the C axis. k is inversely related to the
3
priate ionization energy I of the Lewis base B. In this con-
B
centrifugal distortion constant D in the quadratic approxi-
J
text, the word ‘‘appropriate’’ implies that IB refers to
removal of an electron from the orbital ͑nonbonding -type
or -type͒ directly involved in the intermolecular interaction.
In fact, for a series of ten complexes B¯ICl, the variation
of ␦ with I is well described by the expression ␦ ϭA
mation by the expression33
2
3
0
1Ϫ͑B /B ͒Ϫ͑B /BBrCl͒
PH
3
k ϭ͑16 B /D ͒
͕
͖
,
͑13͒
J
0
0
where B , BPH3, and BBrCl are rotational constants of the
0
i
B
i
complex, the PH subunit, and the BrCl subunit, respec-
3
ϫexp ϪbI , as discussed in Sec. I. It is of interest to ex-
͕ ͖
B
tively, and has been defined in Sec. III D. Values of k
estimated by means of Eq. ͑13͒ for the various isotopomers
of H P¯BrCl investigated are included in Table V. The D
amine whether similar behavior is also manifest in the
B¯BrCl series.
3
J
The Br and Cl nuclear quadrupole coupling constants
PH
3
and B values were taken from Table III while B
appro-
0
aa(Br) and (Cl) are now available for eight complexes
aa
priate to the free PH molecule, as given in Table IV, was
7
35
37
3
B¯BrCl, namely those having BϭCO, HCN, H O,
2
employed. In view of the lengthened BrCl bond within the
38
34
39
36
C H , C H , H S, NH , or PH . Hence, values of ␦
2
2
2
4
2
3
3
i
BrCl
complex established in Sec. III D, B
for the free molecule
for these complexes can be estimated by using the procedure
set out in Sec. III B. The results are shown in Fig. 2, in which
was considered inappropriate and the value of this constant
calculated for the bond length r ϭ2.181(2) Å of BrCl when
s
␦i is plotted against I , as obtained from Refs. 13 and 40. In
B
within the complex was taken instead. In fact, use of the free
each case, we used the angle  ϭ6.0(5)° for reasons set
BrCl
av
molecule value of B
would lead to only small changes in
out in detail in Sec. III B. Although Ar¯BrCl is not among
the estimated k values compared with the errors that arise
the complexes so far investigated, we have assumed ␦
i
from those in D and systematic shortcomings of the model.
J
ϭ0.0 and therefore taken this complex as the reference point.
We note from Table V that k is almost isotopically invari-
The error bars on the ␦ values given in Fig. 2 are those
i
ant within experimental error. The magnitude of k estab-
generated by the assumed range in  . The solid line is the
av
lishes that, according to this criterion, H P¯BrCl is not a
3
curve ␦ ϭA exp
ϭ0.870 (eV) , which has been fitted to the points by the
least-squares method.
͕
ϪbIB
͖
,
where Aϭ327.5 and
b
i
strongly bound complex and that it is comparable in binding
Ϫ1
3
4
35
strength with C H ¯BrCl, and HCN¯BrCl, for ex-
2
4
ample. On the other hand, the intermolecular electron trans-
The corresponding curve for ten complexes B¯ICl, es-
fer ␦i in H P¯BrCl is significantly greater than in
10
3
tablished earlier, is also displayed in Fig. 2. We note that
C H ¯BrCl and HCN¯BrCl ͑see Sec. IV͒.
2
4
some points, particularly those for H N¯BrCl and the CO/
3
HCN pair in both B¯BrCl and B¯ICl, deviate from the
curve fitted to Eq. ͑1͒. This probably arises from the limita-
tions of the Townes–Dailey model when applied in this con-
text. Nevertheless, we can discern a strong family relation-
ship between the B¯ICl and B¯BrCl curves, but we
IV. DISCUSSION
This investigation of the ground-state rotational spectra
of six isotopomers of the phosphine–bromine monochloride
complex has established that the molecule has C3v symmetry
and that the intermolecular bond is formed between P and
Br, with a distance r(P¯Br͒ϭ2.869͑1͒ Å. The interaction of
recognize that, for a given B, the value of ␦ for B¯BrCl is
i
systematically smaller than that of B¯ICl. There are two
obvious reasons for the order ␦ (B¯ICl)Ͼ␦ (B¯BrCl).
i
i
4
1
the two component molecules is not particularly strong ͓k
The first is that the electric dipole moment of ICl ͑1.24 D͒
is greater than that of BrCl ͑0.519 D͒, so that distortion of
ϭ11.5(1) N mϪ1͔ and it has been established that there is a
42
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
216.165.95.79 On: Sun, 07 Dec 2014 01:36:46
Legon
