Matrix isolation study of the products of the interaction between metastable Ar and Kr atoms and chloroform. Photodecomposition of HCCl+2 and HCCl−2

Physics I2 (1976) 51-63 0 North-Holland Publishing Company

Chemical

MATRIX ISOLATION STUDY OF THE PRODUCTS OF THE INTERACTION BETWEEN METASTABLE Ar AND Kr ATOMS AND CHLOROFORM. PHOTODECOMPOSITION OF HCCI; AND HCCI; Marilyn Institute

E. JACOX

for Materials Research,

Received 27 June 1975 Revised manuscript received

Nanbnal

15 September

Bureau of Standards,

Washingron, DC 20234, USA

1975

The emission spectrum ofa microwave discharge through argon and the infrared and ultraviolet spectra of the products of the interaction of HCCI, with the periphery of such a discharge, observed arter rapid quenching in an argon matrix at 14 K, indicate that metastable argon atoms play an important role in matrix isolation experiments using such a discharge configuration to produce free radicals and molecular ions. Exceptionally high yields of HCCI: and HCCl; and a significant concentration of Ccl; were stabilized in thcsc experiments. T’he observations support the earlier assignment of the 1037 cm-t peak to “isolated” Ccl;. Upon exposure of the sample to 370-280 nm radiation, a prominent, slighUy perturbed absorption of CIHCI-appeared at 705 cm-‘. This abs orption was destroyed by 280-260 nm radiation. The relatke stability of both HCCI; and Ccl; upon c~posure of the rample to radiation of wavelength longer than 280 nm is attributed to electron scavenging by the HCCI, in the matrix; this species is present in considerably greater concentration than arc the ion products. Evidence is pre%nted for the photodecomposition of HCCI; at wavelengths shorter than about 280 nm. A marked increase in the concentration of HAri when HCCI; Was photolyzed by radiation of wavelength shorter than 260 nm is consistent with the calculated threshold energy for proton transfer from HCCt: to Ar. The resylts of krypton matrix experiments are also consistent with this mechanism. Unstructured absorption bands near 285 and 2.50 nm are tentatively attributed to CIHCL-and HCCI:, respectively. An increase in the concentration of “nonrotating” H,O. compared to H,O molecules free to undergo rotational transitions, when ions are present in the matrix can be attributed to the electric field producted by ionic interactions,

1. Introduction

Recent infrared studies in this laboratory [l] have demonstrated that HC2 is an important product of the decomposition of acetylene which results when this molecule is introduced near the downstream terminus of a microwave+xcited discharge through argon. The pioneering discharge-flow studies by Prince, Collins, and Robertson [2] suggested that metastable argon atoms would play an important role in such a discharge, a hypothesis later confirmed by Fishburne [3] and applied in a large number of gas-phase studies [4] ; In order to test the hypothesis that metastable argon atoms contributed significantly to the production of HC2 in the matrix experimenk, it was decided to conduct sttidies using a similar experimental configuration but with chloroform as_the molecu1.e interacting

with the discharge. In previous studies of the vacuum ultraviolet photolysis of chloroform in an argon matrix [S] , CCL, and Ccl: were found to be the predominant neutral and cation products when 121.6 nm radiation was used, whereas HCCl2 and HCClz predominated when 106.7 nm radiation was employed. The HCClZ

anion provided the requisite overall charge neutrality at both photolysis wavelengths. Since the 3Pz and 3Po metastable states of the argon atom iie at’ehergies 11.54 and 11.72 eV*, respectively, above the ground state, compared to an energy of 11.62 eV for the 106.7 MI emission line of argon:the appearance of HCCls would be consistent with a decomposition mechanism involving collisions with metastable argon atoms. * Thr+&out th:% paper 1 eV = 806547.9 m-l and’1 kcal= 4.184

Id.

...

., The first, experiment on the discharge decomposition products of chloroform showed a dramatic increase in the absorption previously assigned to ion products, compared to any of the photolysis experiments. In an attempt to. increase our understanding of this and related systems, a series of studies of the photolysis of these discharge products using filtered mercury-arc radiation was conducted. The results of these studies, presented in the following discussion, constitute an interesting sequence of’ion-molecule reactions in solid argon.

2. Experimental

details*

The sources of the normal and isotopically chloroform samples used in these experiments been described

in the previous

publication

enriched have

[5].

Ar : HCCI, samples of mole ratio 200 and 400 were prepared using standard manometric procedures_ In some experiments, 1% CO was also present in the sample, providing a tracer for chlorine-atom production. The cryogenic cell, the discharge configuration, and the infrared spectroscopic equipment were similar to those of the Ar : &Hz discharge experiments [I] . The sample was exposed to the Nernst glower background radiation of the infrared spectrometer only during spectral scans. The resolution and absolute and relative frequency accuracies are estima!ed to be 1 cm-r between 400 and 2000 cm-l and 2 cm-l between 2000 and 4000 cm-l. Selected spectral regions were also studied using decreased slit widths, slower scanning speeds, and greater chart dispersion, resulting in somewhat higher resolution and in relative frequency accuracies of approxjmately 0.5 and 1 cm-‘, respectively; in these two spectral regions. Studies of the emission spectrum of the argon d.ischarge were conducted by substituting the discharge tube for the first observation window of i: closed-cycle helium refrigeration system similar to that used for the infrared studies, so that the discharge tube was along l

Certain commercial instruments and materials are identified in this paper in order adequately to specify the experimental procedure: In no case does such identitktion imply recommendation or endorsement by the National Bureau of .gtandazds, nor does ii imply thnt the instruments or mat&i& iden%ed are necessrity the bzst available for the purpose.

:

the optic axis of the monochromator. In order to duplicate as closely as possible the discharge conditions but to eliminate light attenuation by buildup of a solid argon deposit, the cold substrate window was removed from the cell, which was maintained at 14 K to remove discharged argon from the system by condensation on the sample block. Both the emission spectrum of the argon discharge and the absorption spectra of solid deposits were observed using an evacuated 0.8-m Ebert-Fastie scanning monochromator equipped with a 1200 groove/mm grating blazed at 200 nm, with 50-p slits, and with an EMI 62553 photomultiplier detector. For the absorption studies, a lithium fluoride substrate window was mounted in the sample block, the discharge tube and the sample inlet tube were affixed to a port perpendicular to the optical axis of the instrument, and the interior of the cryogenic cell was rotated by 90” about the O-ring sealing the shroud (which was attached to the entrance slit of the mono-

chromator) to the cooling stage assembly. In this position, the discharge and the sample jet were directed perpendicular to the cold substrate surface. On returning the cooling stage assembly to its original the absorption spectrum of the sample could served using a deuterium background source ZOO-360 nm spectral region and a tungsten the 360-550 run spectral region. All of the absorption

position, be obfor the lamp for

studies were conducted

.., : ._. ;. (

:

I ‘:,

at

14+1K. Studies of the photolysis of the discharge products trapped‘in an argon matrix were conducted using a medium-pressure mercury arc with filters of Corning glass types 3060,0160, and 7740, which transmit visible and near ultraviolet radiation but less than 1% of the radiation of wavelength shorter than 370,300. and 280 nm, respectively. A Coming glass type 7058 filter, which transmits less than 10% of the radiation of wavelength shorter than 260 nm, was also used in some experiments. In other experiments, the mercuryarc radiation was passed directly through the potassium bromide or lithium fluoride observation window. The wavelength cutoff in these experiments is approximately 250 run, the.limit of significant output of the mercury arc.

,..

,:

M.E. Jamx/Ma(rix

irohrion

53

s~~uiy

3. Observations 3.1. Emissiorl spectrum of the discharge The emission spectrum observed between 465 and 415 nm in a typical study of argon excited by a microwave dis?harge is shown in fig. 1, as are the positions of emission lines of Ar 3P, Ar IP, and Arf which possess relatively low excitation potentials [6]. All of the prominent lines in this spectral region can be assigned to Ar IP, and Ar 3P emissions, with significant contributions from all three of the triplet states. Only a few relatively weak lines can be assigned to Ar*. Since only the small fraction of the 104.8 and 106.7 nm radiation which passes through the pinhole or which arises in the extruding Portion of the discharge was accessible to the sample and argon atoms excited to the metastable 3P2 and 3Po states possess lifetimes greater than 1.3 s [7] , it is likely that most of tbe observed products result from collisions with the longlived argon metastables rather than from photoIysis.

1

-Fig.

461

1.

---440

450

Emission spectrum

.L

.

.

-

..___.

410

of argon excited

A.

410 IS

by microwave

dis-

3.2. Infrared spectra Portions of the infrared spectra in which product absorptions appeared in typical discharge experiments on Ar : HCC13 samples are shown in the solid traces of fig. 2. Most of the product absorptions which are present in these spectra are readily identified. The prominent 899 cm-l peak is contributed by Ccl, [8,9], and the 838, 1271,2498, and 2720 cm-l peaks are contributed by HCCIF [S] . When observed using a higher resolution, the very strong peak ar 1038 cm-l possessed two lower frequency satellites, and the three peaks had relative intensities approximately in the ratio 9 : 6 : 1, appropriate for their assignment to HCCl’, rather than to the 1037 cm-’ fundamental of Ccl;. The prese&e of HCCL: was also indicated by the appearance of its absorption at 1291 cm-l [.5] _A relatively weak absorption at 871 cm-l may have been contributed by CC1 [lo]. In the experiment of fig. 2(a) 1% CO was added to the sample to serve as an indicator of the extent of Cl- and H-atom production. The 1877 cm-l peak of dlC0 [ 1 l] was very prominent, whereas ‘Kbe1863 cm-l peak of HCO [12]- was relatively weak. The absorption at 1816 cm-l, contributed by Cl,CO, provided still. further evidence that Cl-atom production pIayed the

115 Fig. 2. Photolytic behavior of Ar : HCCl, samples codepositcd at 14 K with a beam of Ar that bad been passed through a microwave discharge. 4.81 mmol Ar I CO : HCCI, = 200 : 2 : 1 codeposited (a) ~ over period of 300 min with 5.03 mmol discharged. Ar. -_86 min subsequent mercury-arc photolysis. h > 280 nm. ---38 min subsequent mercury-arc photolysis, X > 260 nm. 4:60 mmol A! : HCC$ = 200 codeposited over @I period of 29.5 min with 4.92 mmol discharged AL -78 min subsequent mercury-arc photolysis, .I>ZgOnm.---27 min subsequent mercury-arc phoiolysis, h>250nm.

: .‘:

..

,.

:

‘.

:-

.,

.,

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:_

...

.:.’

-.

:._.

‘,

:

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-

54

M.E. Jac&/Marrix

predominant

role in these experiments. Several broad absorptions’, the most prominent of lhrhich appeared at 705 cm-l, will be considered in the later discussion. The yield of ionic species in the present series of experiments was substantially higher than in &e earlier experiments using an argon resonance lamp 151. In the experiment of fig. 2(a) the peak optical densities of the 1271 cm-l HCCly absorption and the 1291 cm-’ HCCIZ absorption were 0.097 and 0.081 and in the experiment of fig.a(b) 0.137 and 0.11 I, respectively, whereas in the highest yield 106.7 run photolysis experiment these peak optical densities were 0.050 and 0.034, respectively. Since the argon resonance lamp had a 25 mm diameter window whereas the area of the pinhole was only 1 or 2 mm?, the flux of 104.8 and 106.7 nm photons reaching the sample from the body OF the discharge should be very much Iower than in the earlier experiments, again suggesting a predominant role for argon metastables in these experiments. A higher resolution scan of the absorption region near IO38 cm-l provided evidence that this prominent absbrption was not contributed solely by HCCI;, but that some absorption by WI;, with a prominent fundamental at 1037 cm-t, was also present in the discharge studies. In a study of a chloroform sample enriched to 55% in carbon-13, the eak optical density P of the JO12 cm-’ absorption of H 3C35Cl; was 0.360, whereas that of the 1003 cm-’ 13CC14 absorption was 0.066. High resolution scans of the 800-840 cm-l spectral regions gave approximately 9 : 6 : 1 chlorine isotopic intensity ratios for the absorptions of both H 12 Ccl, and H13CC1,, confirming the presence of two eq&alent chlorine atoms. As in the photolysis experiments, the 6 cm-’ carbon-isotopic splitting of the 2498 cm-l peak was clearly resolved. Since a pure C-H stretching vibration near 2500 cm-l is calculated to have a 7.4 &t-t .&ift on carbon-13 substitution, the data are consistent with the assignment of this absorption to a C-H stretching fundamental. High resolution scans of the 2720 cm-l absorption, much more prominent than in the earlier studies, were again attempted. ‘The center of this broad absorption was definitely shifted to lower frequencies than in studies of ordinary IiCCl j sampl+and two maxima separated by 7. c~-t .co&d bSie!y be discerned, consistent with the earlier Lass&nrnent of this absorption to.a combination band i&olvjn& the C-H stretching and a low frequency bendii@ fun’damental. :

:,;.:

-.

isolation study

As is shown in the broken traces of figs. 2(a) and T(b), marked changes occurred on exposure if-the samples to filtered mercury-arc radiation. The peak optical densities of prominent peaks in these two experiments are summarized in table 1. When the sample was exposed to radiation with a 300 or 280 MI cutoff, the absorptions of both HCCI; and HCClz diminished in intensity, but the fractional decrease in the peak optical densities of the HCClt absorptions was significantly less than that for the HCClF absorptions. Such an observation can best be rationalized by postulating the production of another negatively charged species, since overall charge neutrality of the deposit must be maintained. The diminution in the absorption of these two ionic species was accompanied by a dramatic growth in the intensity of the 705 cm-l absorption and by the appearance of a moderately intense, sharp absorption at 925 cm-’ and of a moderately intense, broad absorption at 975 cm-t. None of these three absorptions showed any shifts or splittings in the experiment on a sample enriched to 55% in carbon-13, suggesting that the vibrations which contribute them do not involve significant motion of a carbon atom. The great prominence of the 705 cm:l peak and the failure to observe any change in the 674 cm-l HC13C13 peak excludes the previously suggested possibility that

the carbon-l 3 counterpart of the 705 cm-l might appear near 674 cm-l.

absorption

When the sample of fig. 2(a) was subsequently exposed to radiation with a 260 nm ciltoff, there was a marked decrease in the intensity of the 705,925, and 975 cm-l peaks, and a low frequency shoulder on the 705 cm-l peak, at 697 cm-l _became more prominent. The intensities of the HCClt absorptions decreased somewhat, but those of the HCCly absorptions changed

very little. In contrast, as is shown in fig. 2(b), exposure to the full light of the mercury arc of a sample previously subjected to radiation with a 280 nm cutoff resulted in almost complete destruction of the 705 cm-’ absorption but retention of its 697 cm-l satellite and in marked decreases in the intensities of the absorptions of both HCCl: and HCCI,. The intensity of the absorption between 1035 and 1040 cm-’ remained significant,

but the most prominent peak underwent a definite 1 cm-l downward shift to 1037 cm-‘, and a high resolution sqan indicated that the.chlorine4sotopic ratib,of this remaining abso$tion.was approximately 3 : I_ Both the frequency shift and &contourof‘this

M.E. famr’/Matrix

isohtion

study

55

Table 1

Peak optical densities of product absorptions in typic;rl studies a)of the infrared spectra of Ar : HCCl, samples codepodted with a beam of dizchvged Ar and subjected to iiltered mercury-arc radiation __A.___-_-cm-l

571 697 705 838 847 871 899 904 925 956 915 1036 1271 1291 1816 1863 1877 249s 2720

Eqzriment

A

Experiment

ODinit

0D28D

0.05 1 shb) 0.120 0.233 sh 0.039 0.398 _

0.046 sh 0.456 0.143 Sh 0.022 0.411 _

0.008 0.445 0.091 0.081 0.051 0.061 0.329 0.195 0.217

0.028 0.024 0.058 0.385 O.CM 0.052 0.068 0.043 0.285 0.11s 0.082

ODZ60

0.043

sh 0.157 0.143 sh 0.022 0.438

0.020 0.020 0.276 0.046 0.025 0.070 0.020 0.291 0.110 0.107

ODinit

Assignment

B O%eo

-

-

0.083 0.084 0.324 0.059 0.034 0.292

sh 0.789

0.192 0.043 0.032 0.288 0.046 _

-

at 14 K

0.699 0.137 0.111

0.102 O.S?l 0.072 0.085

-

-

0.352 0.402

0.145 0.184

O&so

0.156 0.130 0.070 0.019 0.034 0.459 sh -

_ 0.254 0.026 0.024 -

clco ClHClhi [CIHCI-] HCCI; cc1 CCI,, HCCI, HA& M[clHCr] CIHCIhi!ClHCl-J HCCI;, Ccl; HCQ; HCCI; aaco HCO CKO HCCI; HCCI;

a) Experiment A: 4.81 mmol AI : CO : HCCI, = 200 : 2 : 1 codeposiled over period of 300 min with 5.03 mmol discharged AI. Sample subjected to 86 min mercury-arc photolysis, A > 280 nm. then to 38 min mercury-arc photolysis, A > 260 nnr. Experiment B: 4.60 mmol AI : HCCl, = 200 codeposited over period of 295 min with 4.92 mmol discharged Ar. Sample subjected to 78 min mercury-arc photolysis, X > 280 nm, then to 27 min mercury-arcphotolysis, h > 250 MI. b) Shoulder. absorption suggested that it was contributed primarily by residual Ccl:. Although in both the 260 and the 250 run cutoff irradiations the 899 cm-l absorption of Ccl3 grew in intensity, its growth was more marked in the 250 rut-rexperiment. A final point of contrast is provided by the appearance of a partially resolved peak at 904 cm-l in the 250 nm cutoff experiment but not in the 260 run cutoff experiment. The intensity of this peak decreased as th,e sample was exposed

to infrared radiation, behavior appropriate for its assignment to HAri [13,14]. Of especial interest is the behavior of the water impurity absorptions at 1591,1609, and 1625 cm-l at various stages in the photolysis of the discharge samples, illustrated in fig. 3. In trace (a) is shown the water absorption pattern of a deposit in which the beam of argon was not subjected to the discharge. The 1625 cm-l peak, assigned to the 1 o-Oo rotational

transition associated with the bending fundamental-of absorption, and the

H,O; was the most prominent

1609 cm-l peak, assigned to the l,, -l_ 1 rotational transition, and the 1591 cm-l peak, close to that assigned to “nonrotating” Hz0 [ 15,161, were approximately equally intenseIn trace (b), the water absorption pattern is shown for a deposit containing an equal number of millimoles of total material taken from the same two samples as in trace (a) but deposited with concurrent microwave discharge of the argon beam The deposition and observation temperatures of the two samples were the same, within the 0.1 K reproducibility of the temperature controller. The only significant peak in the experiment of trace (b) was that of “nonrotating” Hz0 at 1591 cm-l. As is shown in trace (c). after a period of 280 nm cutoff irradiation, the 1625 and 1609 cm-l peaks began to appear, and as shown in trace (d), their intensities were furhter enhanced compared to the “nonrotating’- water absorption after a period of unfiltered irradiation, but the ratio of the intensity of the 1591 cm-’ absorption to that of the 1625 cm--l absorption remained sigriifi-

M.E. hcox.lMarrix

I

IO%

I

I

Fig. 3. Ha0 impurily absorptions in Ar : HCCI, s=unplcs codeposited at 14 K with a beam or Ar. (a] 4.92 mmol Ar : HCCl, = 200 codeposited over period of 266 min with 4.60 mmol undischarged Ar. (b) 4.60 mmol Ar : HCCI, = 200 codeposited over period of 295 min with 4.92 mmol Ar that had been paved through a microwave discharge. (c) 78 min subsequent mercury-arc photolysis, h > 280 nm. (d) 37 min subsequent mercury-arc photolysis. X > 250 nm. cantly greater than that characteristic of the undischarged samplk. -The photolytic behavior of Ar : DCCl,, samples codeposited with a discharged beam of argon is summarized in fig. 4 and in table 2. As in the Ar I HCCI, disiharge studies, the assignments proposed as a result of the earlier photolysis experiments [ 51 are supported. The considerably higher ion yield in the present experiments facilitated a high resolution scan of the 685 cm-l absorption, which showed the 9 : 6 : 1 intensity ratio appropriate for its assignment.to a vibration involving two equivalent chlorine atoms. The contour of the 799 cm-l.absorption was more complex, but on exposure of the deposit to 370 nm dut.off radiatiod a prominent peak at 796 cm-l disappeared, and the remaining absorption showed the --9 : ,6 : 1 intensity ratio expicted for DCCl,. Sitice in all of the Ai : DCC13 experiments the 685 cm-l peak exhibited behaviqr parallel.io.that of the previously assign&I DCCI, absorptions, this absorption may be cbntribiied by the symmetric carbpn-chlorine .st~etchir.g-_fundamental of KCl,. Presumably.the..cbrresponding Fundamental of HCClt , as yet unidentii -fie,d,.is obscured by prominent chloroform absorptip+

-.:

:

:

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isolarion srmiy

:

:

.: _I

‘_ ..,

II 14ll

III0

Fig. 4. Photolytic behavior of Ar : DCCI, samples codeposited at 14 K with a beam of Ar that had been passed through a microwave discharge. 4.06 mmol Ar : JXCl, = 200 codeposited over (a) period of 286 min with 4 49 mmol discharged Ar. --110 min subsequent mercury-arc photolysis. A > 280 nm. --36 min subsequent mercury-arc photolysis, h > 260 nm. 4.28 mmol AI : DCCI,= 200 codcposited over !b) period of 259 min with 3.64 mmol discharged Ar. -_120 min subsequent mercury-arc photolysis, A>280nm. ---42 min subsequent mercury-arc photolysis, -.I > 2.50 nm.

When Ar : DCCI, samples were subjected to 280 nm cutoff radiation, a very-prominent absorption appeared at 471 cm-l. but there was little or no absorption near 705 cm-l. This very large shift in the 705 cm-l absorption.on deuterium substitution requires that ii be assigned t? a predominantly hydrogenatom vibration. Asin the HCCiS experiments, the fractional decrease in the DCCl$ absorptions was somewhat less than that in the DCCI, absorptions at this stage in the irradiatign, suggesting that another anion species was foimed to maintain,the requisite charge

M.E. Jawx/Mntrix

isolation

57

sruiy

Table 2 Peak optical densities of product abscrptions in typical studiesa) of the infrared spectra of Ar with ZIbeam of discharged Ar and subjected to filtered mercury-arc radiation ------_-cm-’

465 471 644 685 799 864 898 961 973 1037 1122 1892 2056

Experiment

Esperiment

A

ODinit

~D*f30

0.062 Cl.068 0.021 0.120 0.250 0.156 0.150 0.321 0.084 0.143 0.508 0.117 0.176 ____ _.-_---.

sh b) 0.438 0.029 0.061 0.089 0.123 0.150 0.181 0.054 0.143 0.327 0.057 0.097

~___

: DCCI, samples codzposited

Assignment

B

OD260

ODinit

OD,ao

OD,so

0.100 0.136 0.060 0.061 0.089 0.076 0.166 0.181 0.084 0.176 0.215 0.066 0.097’

0.068 0.057 0.016 0.104 0.301 c) 0.147 0.140 0.261 0.103 0.141 0.411 0.091 0.146

sh 0.320 0.021 0.041 0.055 0.110 0.155 0.135 0.072 0.156 0.266 0.041 0.077

0.141 sh

ClDCl-’

0.156

DA$

..-.-.. ._ _. __.__-...-.--d Experiment A: 4.06 mmol Ar : DCCL, = 200 codeposited

at 14 K

-.--

0.034 d) 0.212 0.038 0.013 0.126 0.027 d) 0.032. dl

M[ClDCl-1 Lxc1; !XCl; ocq Ccl, DCCI; DCCI, ccl; DCCI; DCc1; DCCI;

over period of 286 min with 4.49 mmol dischared Ar. Sample sub jccted to 110 min mercury-nrc photolysis. A > 280 nm, then to 36 min mercury-arc photolysis, X > 260 nm. Experiment B: 4.28 mmol Ar : DCCL, = 200 codeposited OYW period of 259 min with 3.64 mmol discharged Ar. S;lmplc subjected to 120 min mercury-arc photolysis, A > 280 nm, then to 47, min mercury-xc photolysis, A > 250 nm. b) Shoulder. C) Overlapped by product absorption at 796 cm-l. d) Broad residual absorption. neutrality. The proximity of the 705 and 471 cm-’ absorptions to the 696 and 464 cm-l absorptions first reported in the discharge experiments of Noble and Pimentel [17] is evident. Moreover, partially resolved absorptions were observed at 697 and 465 cm-l, and these shoulders persisted or grew in the later stages of photolysis. Although these bands

were initially assigned to uncharged,

linear CIHCI and

ClDCl, respectively, subsequent experiments [18,19] have suggested that they should instead be assigned to the CIHCI- and CIDCI- anions. The rather small upward shift in these two absorptions in the present experiments may result from a perturbation of the bichloride anion by a neutral or charged species trapped in a nearby site; the vibrational spectrum of the bichloride anion is known to be extraordinarily sensitive to its environment [ 181. The shoulder at 720 cm-l on the broad 705 cm-l peak may also be contributed by the perturbed anion, and the sharp 925 cm-; peak and broad 975 cm-l peak may be contributed by the perturbed anion counterparts of the combination band which appears at 956 cm-l for the “isolated” anion. Presumably the combination band absorption for

CIDCI- , which should appear near 730 cm-l, is obscured by the prominent DCC13 absorption. As is shown in fig.4(a) and in table 2, when Ar : DCC13 samples which had been codeposited with a beam of discharged argon and subsequently exposed to 280 nm cutoff radiation were subjected to 260 nm cutoff radiation, the 47 I cm-l absor#ion of ClDCldiminished markedly in intensity and the absorptions of DCCl; decreased somewhat, but the absorptions of DCClFwere unchanged in intensity, behavior which paralleled that of the undeutazrated samples. On the other hand, in studies in which the sample was exposed to unfiltered radiation, such as that shown in fig. 4(b), a mark&d decrease in the DCCI, absorption also occurred, and the rate of decrease in the DCCl; absorptions was accelerated. The 644 cm-l. absorption of DAri [13,14,19] was present in both the experiment of fig. 4(a) and .that of fig. 4(b):V&en the sample was exposed to 260 nm cuttiff radiation the peak optical

density of this absorption

doubled, whereas when the

sample was exposed to shorter wavelength radiatiqn peak optical density indreased by a factor of almost eight. Since ihe 26O’nrn cutoff of the filter was not

its

M.E. Jacox/Mmix

58

sharp, it is suggested that a new photochemical process resulting in the production of DArz has its onset near 260 MI and that the growth in the DAri absorption in the experiment of fig. 4(a) resulted .from shorter wavelength radiation transmitted by the filter. On unfdtered

isobtion

study

I

I

irradiation of the sample there was a definite growth in the CCl3 absorption and a slight decrease in the Ccl; absorption. A similar studywas also conducted on a Kr : HCCl3 sample codeposited with a beam of krypton which bad

been passed through a microwave discharge. Absorptions due to ionic products were much less prominent, and studies of their intensity changes on subsequent irradiation of the sample were difficult. The most prominent product absorption was that of Ccl3 at 896 cm-t. The combined absorption of Ccl: and HCCl; appeared at 1035 cm-l. ‘Ike peak optical density of the 1284 cm-l absorpti-m of HCCl: was only 0.020. Absorptions of HCClt appeared at 837,1268, 2500, and 2740 cm-l. When the deposit was subjected to 300 MI cutoff radiation, the high frequency shou!der of the 670 cm-’ HCCl, absorption was substan&My broadened, suggesting that in a krypton matrix ClHCl- absorbs at about 672 cm-l, and a weak, broad absorption centered at 937 cm-l appeared. The HCClT absorptions were completely destroyed, but HCCl$ and Ccl; persisted. A peak present in the initial deposit at 854 cm-l, attributable to HKrz [13,14], was unchanged in intensity. On subsequent exposure of the sample to 260 nm cutoff radiation, the 670 cm-l HCC& peak resumed its initial contour, the 1035 cm-l absorption of Ccl’ and HCCl; was halved in intensity, and the 854 cm- ? peak grew slightly in intensity. The spectra obtained in an analogous study of a Kr : DCCl, s&nple codeposited with a beam of &scharged krypton are shown in fig. 5. The DC@ absorption at 1118 cm-l appeared in the initial deposit with approximately the. same intensity .as the Ccl: abstirption.at 1035 cm- I. ~Aisopresent were a second DC@ absorption at 860 cm-l, absorptions of DCCl, at 685,7?8,960,1892, and 2076 cm-‘, the CCIJ absorption at 895 cm-l, and a prominent absorption.at 789 &I-‘. A peak at 607 cmWi, assigned t’o.DKrz-[13,,14], h&l an initial optical density of 0.031. On exposure of the sample to 300 run cutoff radiation; @t&.789cm-l absorption, presumably contri@ted by the same product as that which contributed

:

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,_’

._ ..

_,..

.-.

:.

:.

:

.

..

.,

._-

.;

.:

Fig. 5. Photolytic behavior of a Kr : DCQ3= 200 sample code posi:ed at 14 K with il beam of Kr that had been passed through a microwave discharge. 3.32 mmol Kt : DCCI, = 200 codeposited over period of 263 min with 2.75 mmol discharged Kr. -_125 min subsequent mercury-arc photolysiq A > 300 run. ---30 min subsequent mercury-ax photalysis, A > 260 nm.

the 796 cm-l peak in the Ar : DCCl3 experiments, disappeared. The absorptions of DCClr were almost completely destroyed, whereas the absorptions of Ccl;, DCCl;, and DKri were unchanged in intensity. A promi.nent new absorption at 449 cm-l was assigned to CIDCI-, maintain ing the overall charge neutrality of the sample. When the sample was then subjected to 260 MI cutoff radiation, this ClDCl- absorption also disappeared, except for a low frequency shoulder at 439 cm-l, attributed to ClDCl- trapped in sites in the krypton lattice in which its interaction with nearby neutral or charged species is minimal. The peak optical density of the DKI~ absorption increased to 0.049, and the DCC$ absorption at 1118 cm-l disappeared. The 867 cm-l absorption and a weak absorption at 862 cm-l. probably contributed by CCL were unchanged on photolysis. There was a slight growth in the CCL3 absorption, and the peak optical density of the CCli absorption decreased from 0.082 to 0.055. 3.3. Lntr4violet spectra As is shown in fig. 6, no structured ultraviolet absorption bands appeared in these experiments, but there were several broad absorption maxima. ‘iIe band centered near 312 run persisted on subsequent photolysis of the sample. After exposure.of the sample to: radiation with a 280 mn cutoff, a band appeared near 285 nm. It is uncertain whether this bmd represented a new product or whether it became evident upon phoiolysis of an:

M.E. JncuxfMarrix isohtion srudy

59

matrix [20], it has been suggested that the 1017 cm-l peak, here assigned to Ccl:, is instead contributed by

I

zao

1

250

I

I

JWJ

I

nm

I

350

Fig. 6. Ultraviolet absorption characteristic of AI : HCCI, samples codeposited at 14 K with a beam or AI that had been passed through a microwave discharge. (a) 3.85 mmoI AI : HCCI, = 100 codeposited over period of 395 min with 2.00 mmol discharged AI. (b) 33 min subsequent mercury-arc photolysis. A > 280 nm. (c) 40 min subsequent mercury-arc photolysis, A > 250 nm.

other species which contributed an underlying continuum. On further unfiltered irradiation, the position of this maximum shifted to about 280 nm, and a band maximum previously present near 250 nm disappeared.

4. Discussion

As already noted, the emission spectrum demonstrates the presence of argon metastables in the discharge configuration used for these experiments and indicates that Ar+ and electrons probably play a minor role. The axtraordindrily high yield of HCCl$ also is consistent with a prominent role for argon metastables. The yield of CCl; relative to that of HCCI; in the discharge experiments is considerably greater than in the earlier 106.7 nm photolysis experiments [5]. Hdwever, traces of water may always be expected to lead tq H-atom production in the discharge, and efficient photodecomposition of HCCI, into H + CCI; is known to occur at 12L6 run [9] _In the open system of the discharge, such a process would cascade, with the product‘H atoms iti turn subject to excitation. As a result of.recent &dies of the produCts of the proton bombardment of CQ, isolated in an argon

a Ccl; vibration of the Ccl: ... Cl- ion pair. In that work, a weaker absorption at 1020 cm-’ which disappeared readily on exposure of the sample to radiation in the 500 to 3000 nm spectral region or to electrons from an exposed 10 V filament was attributed to “isolated” Ccl;. Several arguments suggest that the original assignment of the 1037 cm-’ absorption to Ccl; remains valid. In the I21 -6 nm HCCI, photolysis experiments previously reported [S] the primary photolysis products were principally H + Ccl,, and the CC13 was subject to photoionization to produce Ccl;. The. only source of Cl- in those experiments would be dissociative electron attachment to HCC13 to produce HCClz f Cl-. However, there was very little evidence for HCC12 production; apparently cage recombination of HCCl, with Cl- was favored. The significant concentration of HCCl, was attributed to a Cl-atom detachment at low eleciron energies. Even at an Ar : HCCl, mole ratio of 4000 the Ccl< absorption pattern was prominent. Furthermore, in both the photolysis and the argon discharge -xperiments there was no evidence for the stabilization of any CClb. It has been suggested [20] that, even though the first ionic state of Ccl, is some 8 eV above the ground state, there may be a substantial coulombic stabilization of a C.&J --. Cl- ion pair, with a further barrier to the recombination because of the planarity of the Ccl; group. Since in the present series of experiments the 1037 cm-l absorption persisted undiminished when the sample was exposed to radiation of any energy up to 4.75 eV (260 nm) and in the presence of higher energy radiation it disappeared with a simultaneous increase in the Ccl3 absorption, without the appearance of CCI,, it is much more re,asonable to attribute its disappearance to an electron capture process than to the surmounting of ti energy barrier for the return of an ion pair to a covalent ground state.. In recent studies of the photoionizatior. spectrum of HCCl-, with ion detection, Werner, Tsai and Baer [21]

observed the onset of HCCl; production at 11.37 eV and of HCCl$ production at 11.49 eV. The yield of HCCI; was considerably greater than that of HCClz. They did not observe CCI;, which has a calculated appearance potential of 12.29‘eV. In the earlier studies of the photolysis of HCCI, by argon resonance radiaiion [5], we attributed &production of prominent HCf$ ‘.

absorptions

to the secondary photoionization

of

HCCl2, which was observed in relatively low concentration. The results of Werner, Tsai and Baer indicate that the reaction HCCll + Ar(3Pn,~) + HCCl~ + Cl + e

(1)

and the direct photolysis of HCQ, by 106.7 nm radiation to produce HCCI; are energetically possible. Because of the low yield of HCCl, in the discharge experiments. Reaction (1) is believed to be the predominant source of HCCli in the argon discharge studies. Conceivably the 796 cm-r peak, which appeared in the DCCI, experiments and disappeared when the sample was exposed to radiation of wavelength longer than 370 run, was contributed by DCCl~. However, data are insufficient for a definitive assignment of this absorption. In the krypton-discharge experiments, the energies ofmost of the excited krypton atoms are expected to range between 9.91 and 10.64 eV, insufficient for direct production of HCCl:. In addition to the expected strong absorption of CC13, a weak to moderately intense absorption

at 971 cm-l

in the Kr : DCCl3 experiments could be assigned to DCClz, as could be a weak absorption at 814 CIII-~ [22] _ The corresponding HCCl, absorptions in the Kr : HCCI, study could not be identified because of strong overlapping absorptions of Ccl, and HCCl,. It appears likely that the production of CC13 from HCC13 is sharply peaked near 121.6 nm (10.2 eV), with an underlying dissocjative continuum for the production of HCCl,. In the krypton discharge experiments, both Ccl; anti HCCl$ are most likely formed by Ionized of the corresponding free radical. The unique behavior of the moderately intense, broad 706 cm-l absorption which appeared in some

of the earlier photolysis experiments was consistent with the suggestion that it might be contributed by HCCls resulting from the capture of “thermal” electrons by chloroform at 14 K IS]. As already noted, it must be reassigned to ClHCl- which is sub.jected to a small site perturbation. In the present experiments, it is presumed that electrons, like at least the lighter atoms,are sufficiently mobile in the argon lattice that virtually all of them arc scavenged by chloroform, forming negative ion pro.ductn which provide- the requisite charge balance of the sample. Since’molecuies cannot diffuse through the argon lattice-at.i4I$, CJHCl- must be formed by a reaction :

: : ... ‘. . :

‘.._

:: ;

.

.-

.

.

involving photodecomposition of HCCll. which disappeared as the ClHCl- absorptions grew, or by the reaction of electrons, Cl atoms, or CI- produced in a

photochemical

process. Possible reactions include

HCCll+hv*HCC12+e

(2a)

followed by HCC13+e-,HCC12+C1-,

(2b)

or CI~+hv~CI+Cl

(3a)

followed by Cl + HCClT + HCC17 -I-Cl-

_

(=I

Of course, some electrons might recombine with Ccl: or HCCl;, but the probability of their encounter with HCC13, present in much higher concentration, is greater. Cl, is expected to be present in the system as a result of the reaction of initially formed Cl atoms. Reaction

sequences (2) or (3) could then be followed by a cage recombination HCCl? + Cl- -s. Cl- ... HCClz + CIHCl- + CC1 . Alternatively, sequence

(4)

ClHCl- may be formed by the reaction

HCCl, + hv --, HCCl + Cl-

@a)

and HCCl, + Cl- -, ClHCL- + CCl,

Oh)

However, reaction (Sa) would be disfavored because of the polarizability of HCCI, leading to cage recombi-

nation of the products. Unfortunately, there are no infrared data on the other product formed together with ClHCl-. It is likely that certain vibrational fundamentals of ionic species are much more.strongly absorbing than are those typical of neutral molecules, because

of the relatively

large oscillations

in the dipole

moment possible for charged species. Thus, the actual concentration of CIHCl- and of other ionic species in

these experiments may well be very low. CC1 was formed in the discharge experiments, but little change in its concentration was generally observed after photolysis.

The most prominent absorption of obscured by the absorption of H13CClj present in natural abundance in the HCC13

Ccl, is; unfortunately, experiments,

as well as bythe

absorption

of DCCl,.

M.E.

Jamx/,tfatrrixisobrion

The very small change in the intensities of the ClCO absorptions when the sample was exposed to radiation of wavelength longer than 280 nm suggests that C’IHCImay be formed by a process not involving significant migration of Cl atoms or Cl- through the argon lattice. This observation suggests, but does not prove, that CIHCL- is formed by the sequence of reactions (?a), (2b) and (4). On 260 nm cutoff irradiation, the 705 cm-l absorption diminished dramatically in intensity, and there was a modest increase in the 697 cm-l absorption of “isolated” ClHCl-. A photoinduced reorientation could possibly account for the growth in the 697 cm-l peak. There was very little change in the peaks assigned to HCCIS, but the HCCl: absorptions decreased in intensity. The decrease-in intensity of both the HCCI; and the CIHCl- absorptions is consistent with the maintenance of overall charge neutrality of the sample. The mechanisms by which these ions were destroyed remain somewhat uncertain. Simple photoelectron detachment from CIHCl- and electron scavenging by HCCIG, as well as by HCCI,, would also impiy electron scavenging by Ccl;. Since the intensity of the Ccl; absorption remained unchanged, it is believed that the rate of photodetachment of electrons was sufficiently low that scavenging by the cations was unimportant. When still shorter wavelength radiation was employed, the decrease in ion products was more dramatic. Both HCCl; and HCCIF disappeared, but some CC15 and “isolated” CIHCI- persisted. There was a dramatic growth in the concentration of HAri. Because of the relatively great stability of DArz, this growth was particularly evident in the DCCl, experiments. The growth in HArj: at wavelengths beyond about 260 nm provides a partial key to the processes characteristic of 280-250 run irradiation. Removal of HCClf from the system could occur not only by electron capture, but also by its photochemical decomposition. Photodetachment of a proton is not energetically possible using radiation of these wavelengths; recent studies of proton transfer by HCClt have led to a value of 202.5 + 0.5 kcal/mol for the proton affinity of CC12 [23]. However, using the recent value of 90 kcal/mol for the proton affinity of argon [24], the

threshold for proton transfer from HCCl; to Ar is readily calculated to lie at 112.5 +- 0.5 kcallmol: corresponding to 254 nm radiation. ArH+ formed by

study

61

such a process would be expected to,form cluster ions with the argon matrix [ 141, accounting for the observed HAri production. Considering the error limits for both proton affinities and-the incomplete cutoff of 260 nm radiation by the fdter. the calculated threshold for proton transfer from HCCL; to Ar is in excellent agreement with the observed threshold for HAri production. Furthermore, disappearance of HCCl: at wavelengths somewhat longer than 260 nm is consistent with the presence of an excited efectronic state of HCCl: in this spectral region, providing a mechanism for mlttating proton transfer. Further support for a photoinduced proton transfer from HCCLS to the argon matrix is provided by the results of the krypton matrix studies. The proton affinity of krypton has been found to be 101 kcal/mol P41, somewhat greater than that of argon. The threshold for proton transfer from HCCI; to a krypton matrix is calculated to be 102 !ccal/mol, which corresponds to 280 run radiation. The initial DKri absorption of the experiment of fig. 5 retained a peak optical density of 0.03 1 when the sample was exposed to 300 nm cutoff radiation. When 260 nm cutoff radiation was used, the peak optical density of this absorption increased to 0.049, suggesting that the threshold for DKri production lies in the 260-300 nm region. Photodissociation of HArz is also possible in these experiments [ 141 and may provide a mechanism for removal of the electron from anions. If an H atom is detached from this species, leaving Arz, conceivably the “hole” in the argon lattice will have at least limited mobility and will be extinguished by electron transfer from an anion. The relatively great stability of HCCI; and its eventual disappearance by a photochemical process rather than by electron capture provide further precedent for the stability of Ccl;. The photolytic stability of the broad maximum near 312 nm in the ultraviolet scans suggests that it is contributed by a relatively stable product such as CCl-,. While there is a well known Cl2 continuum in this spectral region, there was no absorption m_aximum near 312 nm in earlier studies in this laboratory of the spectrum of Ar : Cl, : N&N deposits having an Ar : Cl2 mole ratio of approximateiy 100. In the earlier spectroscopic studies of ClHCl- [ 1 g] , a band at 287 nm was tentatively assigned to this species. The 285 nm band, which shifted to about 280 &II on

M.E. Jacox/h4arrix isobtion study

62

shofiei wavelength photolysis, may also be tentatively assigned to CIHCI- ..The broad band with maximum tiear 250 run disappeared on unfiltered photolysis, suggesting its possible assignment to HCCIZ, which imdergoes photodecpmposition in this spectral region. The_ behavior of the water absorptions shown in fig. 3 is of considerable interest, since the infrared spectrum of water isblated in an argon matrix is extraordinarily sensitive to the temperature and to the presence OF.other molecules in the sample. Because

the proniinence of the Z591 cm-l absorption attribtited to ‘%~onrotating” i-l,0 was greatest when the concentration of ions was a maximum and because the transitions involving changes in the rotational quantum numbers of the water molecule became progressively more prominent as the ion concentration decreased, it is suggested that the high electric field associated with ionic interactions in the matrix inhibits the rotation of H,O. Further support for this suggestion hti been obtained in other experimental studies in ,this laboratory. In a series of experiments on another system in which ion production was known to occur, .outgassing of the recently soldered cryogenic cell led to prominent water absorptions at 3712,3757, and 3776 cm;!, assigned to the Do-l_l. the I_,-Or,, and rotational transitions of the Ye vibrathe 2-2-1-l iional funda&ntal of HzO, respectively [ 15,161, but

the 3735 cm-l band of “nonrotating” Hz0 was relatively weak. Although the high optical scattering typical of infrared

observations

argon discharge

above about 3000

experiments

metric measurements

made accurate

cm-1

in

photo-

difficult, there was a definite

band and a considerable growth in the 3757 cm- 1 band when the sample was irradiated with wavelengths effective in decreasing the i&n concentration. In many other experiments invoftig photoinduced charge transfer from an alkali metal to an electron acceptor mo!ecule in aNargon matrix, the ,.,162s Cm-‘l absorption of I-&O decreased in intensity and the 159 1 cm-l Bbsorption grew- in intensity when t& sample waS exposed to mercury arc radiation, ‘.Ieading tp the appearance of infrared absorptions as--signed to a variety ofionic species. Although other pr@~e,s such’as iiydrogeti bond& interactions &o i&bit the rotation of water.in an grgon matrix; it is suggested that changes.in the relative intensities of tie i!Y I ana.r&zs cm-.G&tes $s+N!+ty &s&pti~* on. itiadi&ionofa &uen’&ple may serve as a useful probe decrease in the 3735 cm-l

for changes in the concentration

of ions in the sample.

5. Conclusions Spectroscopic observations of the discharge and of the matrix isolated free radicals and ions produced by the interaction of an Ar : HCCl, sample with the periphery of a microwave discharge through argon followed by rapid quenching of the products at 14 K indicate that metastable argon atoms play an important role in.such discharge experiments. Although HCCI: is the major cation in the HCCIJ experiments, some CCl; is also produced. Detailed consideration of the processes possible in this system dictates the retention of the earlier assignment of the 1037 cm-l absorption ’ to Ccl:. Exposure of the sample to 280 nm cutoff radiation led to the appearance of prominent absorptions of C!lHCl-, slightly perturbed by interaction with a nearby molecule. Although several reaction sequences may account for the production of CIHCI-, a mechanism involving dissociative electron capture by HCCl, followed by cage recombination of Cl- with HC,C12is favored. Most of the ClHCl- disappeared when the sample was exposed to radiation between 260 and 280 nm. The concurrent decrease in the HCCIZ concentration results from photodecomposition.rather than from electron scavenging; HCCl3, present in comparatively high concentration, is the principal electron scavenger imthese experiments. A marked increase in the concentration of HArA when 250-280 nm radiation was used to photodecompose HCClz corresponds very well with the calculated threshold for proton transfer from HCCl$ to Ar. An increase in the intensity

of the 1591 cm-l

absorption assigned to “nonrotating”

Hz0 in an argon matrix relative to the absorptions associated with low-lying rotational transitions when a high concentration of ions was present is attributed to the influence of the high electric field resulting from ionic interactions in the matrix.

References [l] M.E. Jacox; Chem. Phys. 7 (1975) 424. 121J.F. Prince, C.B. Collins and W.W. Robertson, J. &em. phys. &I@U%?JI~IP.

Pj EA. Fisbbumk J. Cherk Pbys.47.(1967),58.-

I

ME. JacorlMatrir

I41 D.H. Stedman and D.W. Seizer, Prog. Reaction Kin. 6, part 4 (1971). 151 M.E. Jacox and D.E. Milligan. J. Chem. l’hys. 54 (1971) 3935. Interest [61 C.E. Moore. A Multiplet Table of Astrophysical Princeton Univ. Obr Conlr. No. 20 (1945); Natl. Bur. Std. (U.S.) Tech. Note 36 (1959). [‘I R.S. Van Dyke, Jr., C.E. Johnson and H.A. Shugart, Phys. Rev. A5 (19723 991. [81 L. Andrew~ J. Chem. Phys. 48 (1968) 972. [91 E.E. Rogers, S. Abramowitz, M-E. Jacox and D.E. Milligan, J. Chem. F’hys. 52 (1970) 2198. [lOI ME. Jacoa and D.E. Milligan, J. Chem. Phys. 53 (1970) 2688. IllI M.E. Jacox and D.E. hlilligan, J. Chem. Phyr $3 (1965) 866. [I21 D.E. Milligan and M.E. Jacox, J. Chem. Phys 51 (1969) 277. iI31 V.E. Bondybey and G.C. Pimentel, J. Cbcm. Phys. 56 (1972) 3832 1141 D.E. Milliean and M.E. Jacox, J. Mol. Spectry. 46 (1973) 460.

imlattin

study

63

Redington and D.E. M&an, J. Chem. Phys. 37 (1962) 2162. 1161R.L. Redington and D.E. Milligan. J. Chem. Phyr 39 (1963) 1276. 1171 P.N. Noble and G.C. Pimentel. J. Chem. Phys.. 49 (1968) 3165. I181 D.E. Milligan and M.E. Jacox. J. Chem. Phys. 53 (1970) 2034. B.S. Ault, J.M. Grzybowski and R.O. Allen, 1191 J- Andrew

.I151R-1.

J. Chem. Phys. 62 (1975) 2461. I201 L. Andrew% J.M. Gpybowski and R.O. Allen, J. Phys. Chem. 79 (1975) 904. 1211 AS. Werner, BP. Tsai and T. Baer, J. Chem. Phys 60 (1974) 3650. 1221 T.G. Carver and L. Andrew, J. Chem. Phys. 50 (1969) 423.5. Ion [231 S.G. Lhs and P.J. Ausloos. Int. J. Mass Spectram. Phys., in press. in: Interactions Between Ions and 1241 J.L. Beauchamp. hfalecules, cd. P.J. Ausloos (Plenum Press, New York, 1975) p-415.