Matrix isolation infrared and ab initio study of the 1:1 complex between ammonia and carbon monoxide

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 448 (1998) 221-230

Matrix isolation infrared and ab initio study of the 1:1 complex between ammonia and carbon monoxide 1 Jan Lundell a'*, Magdalena Krajewska b, Markku R~is~inena "Laboratory. of Physical Chemistry, University of Helsinki, PO Box 55 (A.I. Virtasen aukio 1), FIN-O0014 Helsinki. Finland blnstitute of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroelaw, Poland

Received 22 September 1997; revised 17 November 1997; accepted 17 November 1997

Abstract

The structure, energetics, and vibrational properties of complexes formed between NH3 and CO have been investigated by matrix isolation FTIR spectroscopy and ab initio molecular orbital theory. Two stable computational minima were found representing nearly linear hydrogen bonds between the subunits. The NH3-CO and NH3-OC species were calculated to be bound by - 3.5 and - 1.7 kJ mo1-1, respectively. Experimentally, both weak complexes could be identified in low temperature argon matrix with the oxygen-attached complex dominating, quite surprisingly, in photolytic complex formation from formamide. © 1998 Elsevier Science B.V. All rights reserved Keywords: Matrix isolation; FTIR spectroscopy; NH3-CO complex; Photochemistry; Ab initio calculations

1. Introduction

The nature of hydrogen bonding and its relationship to other weak forces of molecular association are central themes in several areas of chemical research. Such intermolecular interactions are profound in the phenomena of solvation, chemisorption, phase changes, and in biological molecular recognition. Therefore, it is unnecessary to underline the importance of the hydrogen bond interaction, which has been established as the major noncovalent or nonionic interaction between numerous chemical compounds. Along with the rapid development of

* Corresponding author. E-mail: [email protected] ~Submitted at the XIIth Conference-Workshop Horizons in Hydrogen Bond Research in Nieder6blarn, Styria, Austria, 21-26 September 1997.

sensitive experimental and computational methods, a growing interest has been guided towards the longrange, weak interactions. Recently we have demonstrated that hydrogen bonded complexes can be formed from a suitable precursor [1-4] in various rare gas matrices. UV radiation can be applied to decompose the chosen precursor and the formed photoproducts are trapped by the cage effect introduced by the surrounding matrix. This method has been succesfully used to produce and identify the 1:1 hydrogen bonded complex between formic acid and CO after UV induced photodecomposition of formic acid anhydride [ 1]. Monomeric formic acid was used as a precursor when the w a t e r - C O complex was studied in different rare gas matrices [2,3]. The hydrogen bonded complex H z S - C O could be produced by prolonged UV irradiation using wavelengths longer than 250 nm [4].

0022-2860/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S0022-2860(98)00353-6

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Ammonia is an interesting target in the sense of hydrogen bonding. It is well known in the literature [5] that NH3 forms hydrogen bonds in the condensed phase. However, the gas phase studies, for example, of NH3-H20 [6], NH3-CO2 [7], and NH3-N20 [8], indicate that NH3 has failed in every case to form a classical hydrogen bond, i.e. a hydrogen bond which is either linear or nearly linear. The only gas phase study of the NH3-CO complex [9] in our knowledge suggests a van der Waals interaction between NH3 and CO instead of a hydrogen bond. According to Fraser et al. the interaction occurs from the carbon end of CO to the ammonia nitrogen with a C - N distance of 3.37 ,~. In this paper we will show that after UV induced photodecomposition of formamide a 1:1 complex NH3-CO is formed which is trapped by the surrounding low temperature argon cage. We will also discuss the structural and vibrational properties of the formed complex. We claim that the complex shows an almost linear hydrogen bonded structure in solid argon and that the high level ab initio calculations support this experimental assignment.

2. Experimental details The vapour pressure of the precursor used, formamide (FAM), is too low to allow the compound to be handled by usual vacuum line methods. Therefore, the argon matrices were prepared by flushing a small amount of molecular sieves, wetted with FAM, with argon gas under reduced pressure. The gas mixture thus formed was sprayed onto the cold CsI window kept at 15 K in a Displex DE-202E closed cycle cryostat. Accurate matrix ratios of FAM isolated in an argon matrix could not be obtained by this method of deposition, but we estimate the ratio to be between 500 and 1000. The deposition conditions (speed of deposition, temperature of FAM on sieves during flushing) were optimized to achieve minimal amount of associates in the deposited matrix. All spectra were recorded on a Nicolet 60 SX FTIR spectrometer coadding typically 128 or 200 interferograms. The spectrometer was usually run at 0.25 or 0.5 cm -j resolution in the spectral region of 4004000 cm 1. The photolysis source for the experiments was an excimer laser (Estonian Academy of Sciences, ELI-

76) operating at 193 nm (ArF). Typically, the pulse energies were between 15 and 25 mJ, and the irradiation area on the cold window was ca. 3 cm -1.

3. Computational details All calculations were performed within the framework of the ab initio closed shell approximation using the GAUSSIAN 94 [10] package of computer codes. The complex properties were considered via the supermolecular M¢ller-Plesset perturbation theory to the second order [ 11,12]. Additionally, the complex properties were studied at the quadratic CI level including single and double substitutions [ 13]. The applied basis set was the split-valence, 6-311 type Gaussian function basis set [14,15], added with diffuse [16] and polarization [17] functions on all atoms to give the 6-311 + + G(2d,2p) basis set. Also, this basis set was enlargened to include multiple polarization functions (6-311 + + G(3df,3pd)) to give better flexibility for the complex calculations. In addition to the Pople type basis sets, the complex properties were estimated using the correlation consistent triple-zeta basis set cc-pVTZ of Dunning [18,19] at the MP2 and QCISD levels of theory. The interaction energy of the complex was estimated as the difference of the total energy between the complex and the monomers at infinite distance, where the monomer wavefunctions were derived in the dimer centred basis set (DCBS). This approach corresponds to the Counterpoise Correction (CP) proposed by Boys and Bernardi [20], aimed to minimize the basis set superposition error (BSSE) in the interaction energy. The optimized structures at the MP2 and QCISD levels of theory were further used to evaluate the interaction energies at higher correlated levels ranging from higher order perturbation theory to coupled cluster approach (CCSD and CCSD(T)). Approximating the interaction energy at the CCSD(T) level of theory using MP2 optimized structures has previously been noted to give reasonable estimates of the interactions compared with the experimental values [2,21 l. All calculations were carried out on SGI PowerChallenge computer cluster and CRAY C94 supercomputer at the CSC Centre for Scientific Computing Ltd (Espoo, Finland).

223

J. Lundell et al./Journal of Molecular Structure 448 (1998) 221-230

Fig. 1. The MP2/6-311 + + G(3df,3pd) calculated structure of the carbon-attached NH3-CO complex. Fig. 2. The MP2/6-311 + + G(3df,3pd) calculated structure of the oxygen-attachedNH ~-OC complex.

4. Results and discussion 4. I. Structure

Two minima for the NH3 + CO complex were found on the complex potential energy hypersurface. These structures are shown in Figs. 1 and 2, and their structural parameters are given in Table 1,Table 2. Both predicted structures represent a 'traditional' hydrogen bonded complex, where the hydrogen atom is situated between two electronegative atoms in an almost linear arrangement. This is surprising since in molecular beam experiments [9] only interaction from the carbon end of CO to the ammonia nitrogen could be found. Taking this gas phase structure as a starting point for the optimization procedure resulted in an almost linear hydrogen bonded structure both for N H 3 - C O and N H 3 - O C at the MP216-311 + + G(2d,2p) level of theory. Even though this ab initio result contradicts the structure derived from the gas phase measurements, it is in good agreement with

previous computational work [22]. In their work, Reed et al. reported also the two structures H 2 N H CO and H 2 N H - O C using lower HF/4-31G, HF/631G(d), and MP2/6-31G(d) levels of theory. It is interesting that at all levels of theory in this study the tilt from the linear hydrogen bonded arrangement is just a few degrees. Also, the carbonattached complex N H 3 - C O seems to be more tilted from the linearity than the oxygen-attached complex N H 3 - O C . This can also be seen in Figs. l and 2. At MP2 levels the O - C - H b bond angle for the carbonattached complex deviates by 10° from a linear arrangement, and the C - H b - N bond angle is tilted by 7 ° with the Pople type basis sets. At MP2/ccpVTZ level the tilt for the C - H h - N bond angle is estimated to be only - 1 °, close to the estimate at the QCISD/cc-pVTZ level. For the oxygen-attached complex the C - O - H b is much closer to linear arrangement at the MP2 levels than the corresponding

Table I Calculated properties" of the NH3-CO complex MP2/6-31 l++G(2d,2p)

MP2/6-31l++G(3dL3pd)

MP2/cc-pVTZ

QCISD/cc-pVTZ

r(C-O) r(N-H0 r(N-H:) r(C Hb) r(N-Hh) r(C-NI

1.1360 1.0084 1.0084 2.6361 1.0084 3.6333

1.1348 1.0118 1.0118 2.6176 1.0121 3.6177

1.1380 1.0114 1.0114 2.7193 1.0116 3.7210

1.1308 1.0121 1.0121 2.7900 1.0119 3.7925

/_H ~-N-H 2 / H ~-N-Hb Z_H~-N-Hh Z_(O-C-H b ±C-H b-N

107.1 107.3 107.3 172.8 170.0

106.7 107.0 107.0 173. I 169.6

105.8 106.0 106.0 178.8 170.6

105.8 106.0 106.0 180.9 170.9

E~l [a.u.]

-169.5969395

-169.5955697

-169.590263

-169.6076892

~The bond lengths are in ,~ngstr6msand angles are in degrees. The hydrogenbonded hydrogen is denoted as Hb and the nonbondedhydrogens are denoted as H~ and H2.

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J. Lundell et aL/Journal of Molecular Structure 448 (1998) 221-230

angle in NH3-CO. At QCISD/cc-pVTZ level the C O-Hb angle is predicted to be 172.5 °, and more tilted than for the other levels predicting a tilt of a few degrees. The O - H b - N bond angle is estimated to deviate from linearity by --6-7 ° at all levels. Simultaneously, the oxygen-attached complex shows a smaller interaction distance between oxygen and the hydrogen bonded hydrogen (Hb) of ammonia. At MP2/6-311 + + G(2d,2p) level the O-Hh distance is 2.55 A, which decreases to 2.50 A when additional polarization functions are added. The correlation consistent basis set cc-pVTZ increases the interaction distance close to 2.60 A both at the MP2 and the QCISD levels of theory. For NH3-CO the C-Hb interaction distance is roughly 0.1 A longer than the O-Hb distance in NH3-OC at all levels of theory used. In the oxygen-attached complex the shorter distance allows a larger delocalization of the electron clouds between the two monomer occupied and vacant orbitals. For carbon-attached complex the C Hb distance is longer because of the presence of the 5a antibonding orbital on the carbon atom, which accepts electron density from the electron donor lone pair (n) orbital. This would also favour the n ---, 7r* weak interaction picture presented by Reed et al. [22]. Similar shorter OC complex structure compared with a longer CO complex has been also shown in the case of H 2 0 - C O [23] and HCOOH-CO [2] complexes.

In the gas phase work on NH3-CO, Fraser et al. [9] reported the N - C bond distance to be 3.37 A, and the carbon-attached complex was the only structure observed. Curiously enough, the experimental N-C bond distance is very close to the N-C bond distance found from the theoretical predictions for the hydrogen bonded complex H2NH-CO. At MP2/6-311 + + G(2d,2p) level the N-C distance is 3.63 A, which decreases to 3.62 A when polarization functions are included. Also, similarly to the C-Hb bond, the N - C distance becomes slightly longer when the cc-pVTZ basis set is used. The hydrogen bonded structure clearly predicted by our ab initio calculations could not be found from the gas phase data because the deduced rotational constants implied a hydrogen bond length of 2.055 A for NH3-CO [9]. Obviously this is too short for a hydrogen bond and much smaller than the computational values of ca. 2.5-2.8 A. The discrepancy between the experimental and computational results on geometries suggests further experimental studies. The calculated structural parameters of the subunits change very little from the monomer values upon complexation, which is typical for weak interactions. Moreover, the largest perturbations clearly correlate with the sites of complexation. The N-Hb bond distance increases marginally upon complexation compared with the noninteractive hydrogens in NH3.

Table 2 Calculated properties ~ of the NH 3-OC complex MP2/6-31 l++G(2d,2p)

MP2/6-31 l++G(3df,3pd)

MP2/cc-pVTZ

QCISD/cc-pVTZ

frO-C) r(N-H 0 r(N-H2) r(O-H b) r(N-H~) frO-N)

1.1371 1.0081 1.0081 2.5461 1.0081 3.5491

1.1358 1.0118 1.0118 2.4990 1.0116 3.5049

1.1388 1.0114 1.0114 2.5889 1.0112 3.5957

1.1318 1.0120 1.0120 2.6261 1.0117 3.6320

Z_H I-N-H2 /_ H L-N-H b /-H r N - H b Z_C-O-H b /~O-HrN

107.2 107.3 107.3 178.7 173.2

106.7 106.9 106.9 178.6 172.8

105.9 106.0 106.0 183.3 173.8

105.9 106.0 106.0 172.5 172.8

E~l (a.u.) E,~l (kJ mol ~)

-169.5964072 +1.398

-169.5948242 +1.957

-169.589656 +1.594

-169.6076892 +0.886

aThe bond lengths are in Angstr6ms and angles are in degrees. The hydrogen bonded hydrogen is denoted as Hb and the nonbonded hydrogens are denoted as Hi and H2.

J. Lundell et al.Hournal of Molecular Structure 448 (1998) 221-230

Also, the bending angles in the NH3 subunit are slightly perturbed in the presence of CO. In the carbon monoxide subunit the C - O bond distance is changed at the maximum 0.001 A upon complexation compared with the monomer value. However, for NH 3CO the C ~- O bond becomes larger upon complexation using the Pople type basis sets but for cc-pVTZ basis set the bond is predicted to shorten upon complexation. For NH3-OC the C ~ O bond is predicted to be elongated at all levels of theory.

4.2. Interaction energies The predicted BSSE corrected interaction energies for the NH3-CO and NH3-OC complexes are shown in Table 3. Both local minima show very weak interaction between the complex subunits. For NH3-CO complex the predicted interaction is between 2.7 and 3.5 kJ mol -~ at the MP2 level using any of the basis sets employed in this study. The interaction in the NH3-OC complex is even weaker than in the carbonattached complex, and the interaction is estimated to be close to 1.3 kJ tool -~ at the MP2 level of theory. The highest values for the interaction energies between NH3 and CO are observed at the MP2/6311 + + G(3df,3pd) level of theory, even though the difference (0.4 kJ mo1-1) for the more economical and

225

applicable MP2/6-311 + + G(2d,2p) level is nominal. Also the coupled cluster single-point calculations estimate the interactions both in NH3-CO and NH3-OC to be very close to the MP2 calculated values. From experimental point of view these weak interaction energies are of great importance. In the gas phase experiments [9] it was reported that large amplitude dynamics is occurring in at least 2 ° of freedom, and that the internal rotation of the NH3 subunit about its C3 axis is not sufficient to explain the observation. The calculated interaction energies might give a possible explanation. When the interactions are weak also the rotational barrier for CO between the two complex minima should be quite small. Therefore, in the gas phase at an effective temperature of 10 K, it still might be possible to generate a flipping motion between the two complex structures NH3-CO and NH3-OC. Moreover, if an average structure is determined it might be not a hydrogen bonded one but one that is more like a van der Waals type complex between the two hydrogen bonded structures. In the low temperature solid state the flipping motion over the energy barrier could be expelled, as the matrix environment stabilizes all reactions with an activation energy more than a few kJ mol -~. The formed complex could therefore be 'frozen' by the low temperature and the restricting cage effect. However, it should

Table 3 Predicted interaction energies (in kJ mol -I) of the NH3-CO complex; all values are corrected for the BSSE MP2/6-31 I++G(2d,2p)

MP2/6-311 ++G(3df,3pd)

MP2/cc-pVTZ

-3.095 -2.410 -2.526 -2.268 -2.503

-3.468 -2.830 -2.947 -2.628 -2.843

-2.742 -2.128 -2.257 -2.014 -2.193

-2.266 -2.833

-2.601 -3.260

-1.965 -2.456

-1.384 - 1.475 -1.421 -1.361 - I .399

-1.729 - 1.828 -1.781 -1.680 -1.717

-1.264 - 1.408 -1.347 -1.312 -1.320

-1.419 -1.618

-I.734 -2.031

-1.355 -1.518

QCISD/cc-pVTZ

NH.¢-CO MP2 MP3 MP4D MP4DQ MP4SDQ QCISD CCSD CCSD(T)

-2.872 -2.320 -2.433 -2.220 -2.378 -2.231 -2.177 -2.610

NH 3-OC MP2 MP3 MP4D MP4DQ MP4SDQ QCISD CCSD CCSD(T)

-I.274 - 1.416 -1.358 -1.328 -1.333 -1.356 -1.370 -1.520

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J. Lundell et al./Journal of Molecular Structure 448 (1998) 221-230

Table 4 Calculated harmonic vibrational frequencies (in cm -~) and infrared intensities (in kJ mol -~) for the NH3-CO complex

wI w2 o~ w4 ~ o~ 7 w8 w~ o~Io w II w12

MP2/6-31 l++G(2d,2p)

MP2/6-311 ++G(3df,3pd)

MP2/cc-pVTZ

Frequency

Infrared intensity

Frequency

Infrared intensity

Frequency

Infrared intensity

3681.0 3679.5 3539.8 2124.8 1704.9 1693.8 1063.1 206.9 129. I 73.0 49.2 39.9

35.4 7.1 6.4 36.0 11.1 14.3 150.2 67.0 6.3 0.6 1.8 7.8

3664.8 3664.5 3517.9 2136.5 1669.3 1659.8 1034.6 207.4 126.1 80.4 44.7 36.5

36.3 8.7 7.1 37.0 8.5 11.5 126.9 62.2 5.6 0.4 1.4 7.5

3654.6 3653.3 3512.8 2124.8 1696.4 1687.4 1097. I 150.9 129.4 66.2 40.8 28.7

19.2 2.5 3.0 31.9 11.0 17.2 143.1 49.5 17.0 0.1 2.6 7.1

be remembered that the van der Waals interaction energies of rare gas atoms are usually of similar magnitude and in the solid matrix interactions with the matrix environment might become crucial in the determination of the complex structure within the matrix cage.

in this region at 974.0 and 1031.0 cm -~. In solid Ar the ~'2 absorption of ammonia has been assigned at 976 cm J [24,25], and in the CO matrix three bands were observed at 981.0, 995.5, and 1007.3 cm -j [28]. From the method of preparation of the complex in the

4.3. Experimental Although the infrared spectrum of NH 3 [24-29] has been studied in various matrices, the 1:1 NH3-CO complex has not been identified. However, the work on NH3 in CO matrix [28] gives us a good starting point to the NH3-CO complex. UV irradiated samples of FAM in argon matrix produced several absorptions in the ammonia and CO fundamental vibration regions. To help us with the assignment of the new bands, along with the experimental data on the monomers in matrices, we calculated the vibrational spectra of the complex species using the optimized complex structures. These vibrational frequencies are given in Table 4,Table 5 for the NH3-CO and the NH3-OC complexes, respectively. Computationally, the most intense band for the NH3-CO complex is the w7 band, being between 1000-I 100 cm -~ at all levels of calculations. This band corresponds to the the u2 absorption of the ammonia monomer. The infrared spectra after UV induced photodecomposition of FAM are shown in Fig. 3. As seen, two distinct absorptions are present

Q L) C 0 u) ..Q

,<

~

11 O0

.

~

(a)

I

I

1000

900

Wavenumber (cm-1) Fig. 3. The 900- I 100 cm -~ region of ammonia complexes with CO in solid argon (a) after deposition of FAM at 18 K, (b) after 193 nm irradiation of the precursor, and (c) after annealing cycles 18-3018K.

227

J. Lundell et al./Journal of Molecular Structure 448 (1998) 221-230 Table 5 Calculated harmonic vibrational frequencies (in cm ~) and infrared intensities (in kJ tool -~) for the N H 3 - O C complex

wI co2 co~ 604 cos o~6 co7 cos co~ co ~o co t l col2

MP2/6-311 ++G(2d,2p)

MP2/6-311 ++G(3df,3pd)

MP2/cc-pVTZ

Frequency

Infrared intensity

Frequency

Infrared intensity

Frequency

Infrared intensity

3686.6 3684.0 3544.6 2l 17.8 1701.3 1693.7 1056.8 147.6 103.6 59.4 39.4 38.2

22.8 8. I 2.7 43.4 14.1 15.4 160.9 63.9 3.8 0.2 6.4 3.3

367 I. 1 3665.7 3521.6 2129.2 1665.5 1660.3 1031.4 150.3 96.4 69.6 35.2 34.4

25.5 8.3 3.2 44.9 10.8 12.4 34.4 60.4 3.9 0.0 4.0 3.1

3659.7 3655.2 3515.6 2119.3 1691.5 1687.1 1090.2 88.3 85.9 59.7 21.7 12.3

11.8 3.1 1.3 39.6 12.8 16.1 143.5 52.8 15.1 0.1 1.6 8.7

matrix we know that the majority of CO is trapped in the same cage with ammonia, and we can we get insight on the possible structure of the complex observed from the calculated vibrational shift induced by the interaction. The calculated vibrational shifts for NH3-CO and NH3-OC are collected in Table 6.

!°2

According to these numbers the I.,2 absorption of NH3 should undergo an almost negligible shift in the case of H2NH-OC and a larger shift to higher wave numbers in the case of of H2NH-CO. The experimental shift for the band centre is ca. 2 cm -~ from the monomer value of 976 cm -~ [24,25]. This would imply the existence of the H z N H - O C complex as a photoproduct of FAM in solid At. Nevertheless, it can be noted that the observed v2 band of ammonia

Table 6 Calculated vibrational shifts (in cm ~) upon complexation for the various vibrational fundamentals of the complex subunits NH3 and CO

@

o E

MP2/cc-pVTZ

MP2/6-311

MP2/6-311

++G(2d,2p)

++G(3df,3pd)

-2.4 -3.4 -3.0 +5.2 +11.2 +0.1 +6.0

-1.6 -1.2 -0.7 +6.1 +9.2 +0.1 +3.9

-I.0 - I .6 +0.6 +2.7 +9.7 +1.1 +9.9

+3.2 +1.1 + 1.8 -1.8 +7.6 0.0 -0.3

+4.7 0.0 +3.0 -1.2 +5.4 +0.6 +0.7

+4. I +0.3 +3.4 -2.8 +4.8 +0.8 +3.0

NHs-CO 6o I

o xl

<

_ ___~I

(a)

w2 w3 co4 co5 co6 co7

NH~-OC I

I

2200

2100

Wavenumber(cm-1) Fig. 4. The CO stretching region in solid argon (a) after 193 irradiation of FAM, (b) after annealing to 30 K, and (c) after annealing cycles, spectrum recorded at 19 K.

co i co2 w3 co4 co5 ¢% (-o7

228

J. Lundell et al./Journal of Molecular Structure 448 (1998) 221-230

shows a broad structure with a long tail to higher wave numbers as seen in Fig. 3. Also, some very weak satellite absorptions around 9 8 0 - 1 0 0 0 c m -1 can be seen. These bands are close to the absorptions reported for ammonia in solid CO [28], and they could be due to (NH3)m-(CO)n clusters due to the small amount of FAM dimers in the matrix before UV irradiation. The above assignment would also imply that the 1031.0 cm -~ absorption would not be due to ammonia-CO binary complex but an impurity or aggregation inthe matrix. The question of the existence of HNH2-OC as the major product after FAM photodecomposition is puzzling because this structure represents a higher energy species according to the calculations. In the case of H C O O H - C O [1], H 2 0 - C O [2], and H2S-CO [4] only the computationally lowest energy (carbonattached) species were observed in solid Ar. However, the predicted interaction energies for NH3-CO and NH3-OC are within 1 kJ mol -I from each other and very small overall, so that the surrounding rare gas cage becomes more dominating for the prevalent structure. For H 2 0 - C O in xenon the oxygen-attached complex was the prevalent structure after photodecomposition of formic acid, and the complex could be relaxed to the lowest energy, carbon-attached structure by annealing [3,4]. The carbon monoxide fundamental region is shown in Fig. 4. Immediately it is seen that no sharp bands exist for H 2 0 - C O with a well-defined complex structure [2,3]. Now the CO strecthing absorption is a broad absorption with several bands overlapping with each other. The main component is found at 2136.6cm -~ with smaller maxima at 2143.9 and 2145.9cm -I. The CO monomer in argon matrix absorbs at 2138.6 cm r [29]. The observed shift of the main absorption of - 2.0 cm -1 is in very good agreement with the calculated estimates ranging from - 1.2 to - 2.8 cm -I for the NH3-OC complex. For the NH3-CO complex the calculations show a shift to higher wave numbers from 2.7 to 6.1 cm -~ from the monomer value. Indeed, the weaker bands at 2143.9 and 2145.9 cm -I are well suited to be due to the carbon-attached complex H2NH-CO. However, the broad band structure of the C --- O stretching mode indicates a nonlocalized structure. This is also what we found from the theoretical calculations: a shallow potential energy surface with two structural

minima, the carbon-attached NH3-CO and the oxygen-attached NH3-OC divided by a low energy barrier. The broad structure must be then due to the sampling of different structural arrangements possible in the cage, which are almost isoenergetic and induce an almost equal shift in the C = O stretching absorption. Annealing the matrix (see Fig. 4(b)) shows an increase in the intensity of the carbon-attached complex bands in the CO fundamental region but the cage again favours the oxygen-attached complex at 18 K. The NH3-13CO counterpart of the 2136.6 cm -j band can be found at 2090.0 cm -1. These bands, along with all new absorptions observed after the UV induced photodecomposition of FAM, are collected in Table 7. According to the calculations the ammonia, u2 and the CO fundamental modes are the most intense and convenient fingerprint regions for the complex between NH3 and CO. The calculated c01 mode shows an almost equal intensity with the CO mode but the region is often obscured by, for example, water impurity. Also, ammonia is known to rotate in the matrix at our experimental temperature [24-26] and the N - H stretching absorptions become broader. Table 7 The observed absorptions after 193 nm induced photodecomposition of formamide in solid argon 3607.9 3594.3 3561.0 3417

NH3 ( + CO)

3325.2 3296.8 3284.0 3181.4

NHa ( + CO)

2265.7 (sh) 2263.9 2259.2

HNCO ( + H2)

2145.9 2143.9 2136.6

CO ( + NH3)

2090.0 2048.5 1031.0 974.3

NH3 ( + CO)

13CO ( + NH3)

NH3 ( + CO)

J. Lundell et al./Journal of Molecular Structure 448 (1998) 221-230

However, some new bands appear in the N - H stretching region after photolysis of FAM which can be attributed to NH3-CO interaction. These new bands are at 3325.2, 3296.8, and 3284.0 cm -1. Previously, t h e P l and P3 bands of NH3 in Ar have been observed at 3329 and 3430 cm -l [27], respectively. In CO matrices the u~ band is reported to be at 3320.3 cm -1 while the ~3 band lies at 3427.8 cm -l [28]. Comparing the results from these two matrices indicates a small shift form Ar to CO matrices, and according to the calculations a small shift for the two modes should be observed upon complexation as well. In general the shifts are predicted to be just a few wave numbers. The observed bands at ca. 3300 cm -~ would be in the correct spectral region for ammonia complexed with CO, but the assignment is more or less tentative. The same stands for the weak, broad absorption observed at 3400-3420 cm -~ with a band centre at 3417 cm -~. However, it must be remembered that the many-body interactions are prevalent in the bulk, and may change the interaction drastically compared with the isolated 1:1 complex. The other fundamental absorptions are computationally predicted to be too weak to identify them from the spectra. All observed bands due to the ammonia-CO complex are marked in Table 7. In the 1600-1700 region no bands could be resolved, even though there are indications that in xenon matrices the NH3-CO complex absorbs at ca. 1615 cm -I [301. An interesting addition to the vibrational spectra of the NH3-CO complex is the very strong predicted intermolecular mode at ca. 200 cm -j. This band is predicted to be twice as intense as the v3 absorption of NH 3 and the CO stretching mode. For future studies in the far infrared region this band represents an ideal candidate for the third fingerprint of the NH3-CO complex.

matrix. Computationally two hydrogen bonded structures were also found, as well as a global minimum, carbon-attached complex H2NH-CO and a local minimum, oxygen-attached complex H2NH-OC. Only about 1 kJmo1-1 energy difference in the interaction energies could be found for the two complex structures. Contradicting the expectations according to the calculations, the higher energy species H2NH-OC was found to be the primary photoproduct, probably due to energetically comparable interactions with the cage atoms.

Acknowledgements The CSC Centre for Scientific Computing Ltd is thanked for the time spent on the computer mainframes.

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5. Conclusions The complex between NH3 and CO has been studied after UV induced photodecomposition of formamide. Experimentally broad bands for the complex were observed indicating weak interactions on a shallow potential energy surface. Two possible complex structures were addressed to exist in the

229

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