Vibrational Spectroscopy 16 Ž1998. 85–88
Short communication
CO PPP H 2 O bonding in and on porous ices
A. Givan a , A. Loewenschuss
a
a,)
, C.J. Nielsen
b
Department of Inorganic and Analytical Chemistry, The Hebrew UniÕersity of Jerusalem, Jerusalem 91904, Israel
b
Department of Chemistry, UniÕersity of Oslo, Blindern, N-0315 Oslo, Norway
Received 24 July 1997; accepted 20 October 1997
Abstract
Spectra of H 2 O in solid CO are presented and compared to results obtained for CO adsorbed on different water ices. By
the shape of the n ŽOH. features, CO association to well separated dangling OH bonds and to water polymeric structures may
be distinguished. It is suggested that the CO clusters, formed by CO deposition onto ices of bulk porosity, surround narrow,
hair like protrusions of the ice voids. q 1998 Elsevier Science B.V.
Keywords: Infrared spectrometry; Water ices; CO)H 2 O complexes; CO adsorption
The infrared absorption of the CO stretching mode
was recently shown to be a sensitive indicator of the
morphology and porosity of pure and mixed ices
w1–3x. The attachment of CO molecules to H 2 O in
these ices results from diffusion induced by warming
to T ) 30 K of a CO layer deposited onto them. For
this complexation two cases may be distinguished:
ŽA. An ice prepared by depositing a 4:1 Ar:H 2 O
mixture at 5 K demonstrated, in addition to the main
coupled ‘3 m’ band, infrared features at 3704 and
3720 cmy1 corresponding to uncoupled or ‘dangling’ O–H bonds of 2-coordinated water molecules
Žone hydrogen bond Õia O and one Õia H. and
3-coordinated water molecules Žtwo hydrogen bonds
Õia O and one Õia H. w4x. When such an ice was
)
Corresponding author. Tel.: q972-2-6585313; fax: q972-26585319; e-mail: loewena@vms.huji.ac.il.
covered with a solid CO layer and subsequently
warmed to above 30 K, two new relatively sharp
infrared bands appeared at 3692 and 3629 cmy1 and
were assigned to CO)H 2 O species. A similar doublet, attributed to the OC)H–OH complex, was
observed in rare gas matrices w5–13x, e.g. at 3723
and 3627 cmy1 in argon w5x. The COrice features at
3692 and 3629 cmy1 were interpreted in terms of
bonding of CO to the free, dangling OH bonds on
the microporous surfaces of the deposited ice w1x
Žsurface porosity’’.. The fact that the doublet splitting retains a magnitude similar to the n 3rn 1 frequency difference in the H 2 O monomer, is an indication of CO bonding to distinct, well separated OH
bonds. Temperature induced intensity changes of
these bands followed that of the analogous n ŽCO.
band at 2153 cmy1 . The latter is blue shifted from
the n ŽCO. peak at 2138.5 cmy1 and the CO)H 2 O
dimer band at 2149 cmy1 observed in argon matrices
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 9 2 4 - 2 0 3 1 Ž 9 7 . 0 0 0 4 4 - 1
86
A. GiÕan et al.r Vibrational Spectroscopy 16 (1998) 85–88
part at 2148 cmy1 . This behavior was ascribed w2,5x
to CO clusters distributed in the ice bulk pores,
without specifying the exact nature of the
CO PPP H 2 O bonding. Similarly, ices produced from
premixed COrH 2 O also showed only a single band
attributable to CO)H 2 O species in the OH stretching region at 3623 cmy1 . The temperature dependence is also similar: Upon warming, it red shifted to
3618 cmy1 along with its n ŽCO. counterpart at 2150
cmy1 w2,13,14x.
In the following we discuss our results of experiments involving the trapping of H 2 O species in solid
CO and relate them to the two cases presented
above. The present results are summarized in Table 1
together with relevant previous assignments. Experimental details follow those given elsewhere w2x. In
short, spectra of gaseous H 2 OrCO mixtures, de-
Fig. 1. The infrared spectrum of the free OH’’ region of ices
produced by deposition of a 4:1 NerH 2 O mixture. ŽA. As
deposited at 5 K. ŽB. After warming to 30 K to release the Ne,
then recooling to 5 K, followed by covering the sample with a CO
deposition, and subsequently warming to 30 K to release the
external non-bonded CO. Spectrum recorded after recooling to 5
K. ŽC. After additional warming of sample B to 40 K and
recooling to 5 K.
w5x and thus both spectral regions indicate the possibility of CO bonding to two OH bonds.
ŽB. An ice produced by release of neon from a
mixed NerH 2 O deposited layer, was shown to possess properties of bulk porosity w2x. As produced, it
demonstrated only remnants of the uncoupled OH
bands at 3720 and 3704 cmy1 ŽFig. 1A.. Upon CO
deposition onto this ice at 5 K, followed by warming
to 30 K, a single new broad band, ascribed to
CO)H 2 O species, appeared at 3670 cmy1 ŽFig. 1B..
Further warming to 40 K ŽFig. 1C. red shifted this
absorption to 3640 cmy1 . At 50 K this band completely disappeared together with its n ŽCO. counter-
Fig. 2. The infrared spectrum of the n ŽOH. stretching region of a
sample produced by deposition of a 10:1 COrH 2 O mixture at 5
K. ŽA. As deposited at 5 K. ŽB. Warmed to 26 K and recooled to
5 K. ŽC. Warmed to 33 K and recooled to 5 K.
A. GiÕan et al.r Vibrational Spectroscopy 16 (1998) 85–88
87
Table 1
Summary of band assignments
Band Žcmy1 .
Assignment
Ref.
3720
3704
3692, 3629
3723, 3627
3670 Ž3640.
3623 Ž3618.
3692.7, 3674.2, 3609
3643, 3400
3250
2153
2150
2149
2138.5
1638
1610.5
‘dangling’ OH of 2-coordinated water molecules on ice surface
‘dangling’ OH of 3-coordinated water molecules on ice surface
n ŽOH. of CO)H 2 O species on ice surface
n ŽOH. of CO)H 2 O species in argon matrix
n ŽOH. of CO)H 2 O species on bulk porous ice ŽT shift.
n ŽOH. of CO)H 2 O in COrH 2 O mixed ice ŽT shift.
n ŽOH. of H 2 O molecular species Ždimers. in CO matrix
n ŽOH. of H 2 O polymers in CO matrix
n ŽOH. of amorphous ice
n ŽCO. of CO)H 2 O on ice surface ŽCO attached to two OH bonds.
n ŽCO. of CO)H 2 O in COrH 2 O mixed ice ŽT shift.
n ŽCO. of CO)H 2 O dimer in argon matrix
n ŽCO. in argon matrix
n 2 of H 2 O in water polymeric species in CO matrix
n 2 of H 2 O in molecular species Ždimers. in CO matrix
w4x
w4x
w2,3x
w5x
w2x
this work
this work
this work
w15x
w2,3x
this work
w5x
w5x
this work
this work
posited at 5 K and slowly Ž1 Krmin. warmed up to
34 K Žthe CO sublimation temperature., were
recorded on a Bruker IFS-88 interferometer, coadding
128 scans, with resolutions of 0.5 to 2 cmy1 . The
OH stretching region of a typical sample Žseveral
percent H 2 O in the CO matrix, Fig. 2A. reveals
spectral features which may roughly be divided into
two groups: The sharper and higher peak bands in
the 3700–3600 cmy1 region Žtwo narrow lines at
3692.7 and 3609 cmy1 and two broader ones at
3674.2 and a shoulder at 3658 cmy1 . and the much
broader features in the 3550–3300 cmy1 region,
centered around 3360 cmy1 and with a peak at 3500
cmy1 . Upon annealing the sample to 26 K ŽFig. 2B.
the resolved structure of the first group disappeared
leaving a 35 cmy1 wide feature at 3652 cmy1 and a
200 cmy1 wide band centered around 3400 cmy1 .
While this general picture did not essentially change
upon warming to 33 K ŽFig. 2C., the narrower band
red shifted to 3643 cmy1 , and a new feature around
3250 cmy1 gradually emerged. The much weaker n 2
bending region presents a similar transition from
narrow well resolved absorptions at 1610.5 and 1632
cmy1 at 5 K ŽFig. 3A., to a single very broad feature
centered at 1638 cmy1 ŽFig. 3B.. The sharp features
at 3692.7, 3674.2, 3609 and 1610.5 cmy1 are attributed to distinct H 2 O species Žmostly dimers. w12x
in a CO matrix, while the broader features at 3643,
3400 and 1638 cmy1 are attributed to higher H 2 O
Fig. 3. The n 2 ŽH 2 O. bending mode region of a sample produced
by the deposition of a 10:1 COrH 2 O mixture at 5 K. ŽA. As
deposited. ŽB. Warmed to 26 K and recooled to 5 K.
88
A. GiÕan et al.r Vibrational Spectroscopy 16 (1998) 85–88
polymers. The 3250 cmy1 absorption is typical of
ŽH 2 O.as w15x.
Our new results presented here for H 2 O in solid
CO clarify the attachment of CO within ices of bulk
porosity containing CO clusters distributed within its
pores Žcase ŽB., above.. The 3640 cmy1 infrared
band resembles in shape and position the 3643 cmy1
absorption of polymeric H 2 O species trapped in
solid CO. In comparison with case ŽA. and ŽB., it is
clear that of the two distinguishable types of
CO PPP H 2 O bonding, the CO clusters inside the
bulk pores are not bonded to separated OH bonds,
but rather to ice structures resembling H 2 O polymers. It is therefore suggested that the CO clusters
surround narrow, hair like protrusions of the ice
voids, as suggested by Laufer et al. w16x, producing
an infrared spectrum similar to that of H 2 O polymers
trapped in CO.
Acknowledgements
A. L. acknowledges a Visiting Scientist scholarship from the Norwegian Research Council.
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