Complex between Formic Acid and Nitrous Oxide: A Matrix-Isolation

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Complex Between Formic Acid and Nitrous Oxide: A Matrix-Isolation and Computational Study Luis Duarte, Iiris Rekola, and Leonid Khriachtchev J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09586 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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The Journal of Physical Chemistry

Complex between Formic Acid and Nitrous Oxide: A Matrix-Isolation and Computational Study Luís Duarte, Iiris Rekola, and Leonid Khriachtchev* Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland

ABSTRACT: The complex of formic acid (FA, HCOOH) with nitrous oxide (N2O) was studied experimentally and computationally. Eight structures of the trans-FA···N2O complex and nine structures of the cis-FA···N2O complex were found at the DFT (M06-2X and wB97XD), MP2(full), and CCSD(T)-F12a levels of theory. Two structures of the trans-FA···N2O (1t and 3t) complex and two structures of the cis-FA···N2O (1c and 3c) complex were identified by infrared spectroscopy in an argon matrix. Structure 1t with the bonded OH group appears in the matrices after deposition. Structures 3t and 3c with the free OH groups were prepared by vibrational excitation of the trans-FA conformer in structures 1t and 3t, respectively. Structure 3t is thermally unstable and relaxes to structure 1t at ~15 K. Structure 1c with the bonded OH group is made by vibrational excitation of trans-FA monomer combined with thermal annealing. The lifetimes of the cis-FA···N2O complex structures 1c (81.5 min) and 3c (18.7 min) in an argon matrix at 4.3 K are longer than that of cis-FA monomer (6.3 min). This difference agrees with the calculated stabilization barriers of these species.

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1. INTRODUCTION Formic acid (HCOOH, FA) and nitrous oxide (N2O) participate in atmospheric chemistry. Formic acid is one of the major contributors to rainwater acidity in polluted and remote regions.12

Nitrous oxide has a global warming potential nearly 300 times that of carbon dioxide and is one

of the largest contributors to ozone depletion in the stratosphere.3-4 Recent studies indicate that the FA and N2O annual emissions from known sources may have been significantly underestimated and suggest that these species might be more important to atmospheric chemistry than previously believed.5-6 In addition, FA and N2O have been detected in interstellar molecular clouds and are relevant to studies of prebiotic species in astrochemistry.7-9 The interest in noncovalent interactions of FA and N2O is considerable.10-21 The formation of weakly bound complexes can cause changes in the stability and reactivity of the constituent monomers.22-24 Quantum mechanical tunneling observed in FA is an exciting phenomenon that can affect the dynamics of the reactions and open new chemical reaction channels.25-26 Matrix-isolation infrared spectroscopy is a powerful method to study weakly bonded species,27-29 and a fair number of the FA and N2O complexes have been prepared in cryogenic matrices.30-45 The conformational change is another successful direction of research at low temperatures. The higher-energy conformer of formic acid, cis-FA, can be prepared in cryogenic matrices by selective vibrational excitation of the ground-state conformer, trans-FA (see Scheme 1 for the structures of these conformers).46-50 The cis-FA molecules isolated in cryogenic matrices are not stable and they convert back to trans-FA by hydrogen-atom tunneling.49-51 The cis-FA complexes can be made by using selective vibrational excitation of trans-FA often combined with annealing of the matrix. In this way, the complexes of cis-FA with H2O, N2, CO2, Xe, and several trans-cis and cis-cis (the latter of cis-HCOOD) dimers have been prepared.33-35,


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38, 43, 52-54

This complexation can stabilize the higher-energy conformer. Remarkably, the

conformation-dependent FA + O reaction was found in a krypton matrix.55

Scheme 1. Structures of trans- and cis-FA.

In the present work, we study the complexes of the trans-FA and cis-FA conformers with N2O. These complexes are identified by using infrared spectroscopy in an argon matrix supported by extensive quantum chemical calculations. Selective vibrational excitation of transFA is used to prepare higher-energy structures of the FA···N2O complex. Hydrogen-atom tunneling decay of the cis-FA···N2O complex is measured and discussed based on the calculated stabilization barriers.

2. MATERIALS AND METHODS 2.1. Computational details. The DFT (M06-2X56-57 and wB97XD58 functionals) and MP2(full)59 quantum chemical calculations were carried out with the Gaussian 16 program.60 The calculations employed a very tight optimization convergence criteria and the standard augcc-pVTZ basis set.61 The DFT calculations used the Gaussian 16 default ultrafine integration grid. The explicitly correlated CCSD(T)-F12a calculations were performed with the MOLPRO program.62-63 The cc-pVDZ-F12 (VDZ-F12) correlation consistent basis set was used together with the recommended value of the geminal Slater exponent (0.9)64 and with the Molpro default


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auxiliary basis sets. The geometry optimizations were followed by harmonic frequency calculations at the same levels of theory, which also gave the zero-point vibrational energies (ZPVE) and verified the nature of the obtained minima. The interaction energies were calculated as the difference of the total energies of the complex and isolated species (with the geometries in the complex) and corrected for the ZPVE and the basis set superposition error (BSSE).65

2.2. Experimental details. The FA/N2O/Ar mixtures were prepared with concentration ratios of ~1/1/1000 and 1/3/1000. FA (≥98%, Merck) was degassed by several freeze-pump-thaw cycles. N2O (99.9999%, AGA) and argon (≥99.9999%, AGA) were used as supplied. Prior to the mixture preparation, the glass bulb was passivated with FA vapors by several fill–keep–evacuate cycles. The gas mixtures were deposited onto a CsI window held at 15 K in a closed-cycle helium cryostat (RDK-408D2, Sumitomo Heavy Industries, Ltd.). The matrix thickness was ~100 µm. The FTIR spectra in the 4000−500 cm−1 range were measured at 4.3 K with a Bruker Vertex 80 spectrometer by co-adding 500 scans at a spectral resolution of 1 cm−1. In the kinetic measurements, a long-pass filter that transmitted below 1850 cm−1 was inserted between the Globar source and the cryostat in order to eliminate the light-induced cis-to-trans conversion.51 The Globar light was blocked between the measurements. Conformational changes were promoted by an optical parametric oscillator (LaserVision) that produced infrared light with a pulse duration of ~5 ns, line width of ~0.1 cm−1, and repetition rate of 10 Hz.

3. RESULTS 3.1. Computational results. To probe the potential energy surface of the complexes, several initial geometries were analyzed at the DFT (M06-2X and wB97XD) and MP2 (full)


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levels of theory. Each of the stable minima was then optimized at the CCSD(T)-F12a level. As a result, eight trans-FA···N2O and nine cis-FA···N2O structures were found at different levels of theory. The calculated complex structures with the interaction energies are presented in Figures 1 and 2 (see also Tables S1 and S2 in the Supporting Information). Structures 1t/8t, 2t/7t, 3t/5t, and 4t/6t of the trans-FA···N2O complex are stabilized by O−H···O, O−H···N, C−H···O, and C−H···N interactions, respectively. With the exception of 7t (C1 symmetry), all other transFA···N2O structures are planar (Cs symmetry). The interaction energies of the three most stable structures 1t, 2t, and 3t, are −15.9, −10.6, and −9.6 kJ mol−1 (after ZPVE and BSSE corrections) at the CCSD(T)-F12a/VDZ-F12 level of theory. The O−H···O, O−H···N, and C−H···O bond lengths for structures 1t, 2t, and 3t are 2.02, 2.20, and 2.64 Å (Table 1). Two trans-FA···N2O structures, similar to 1t and 2t, were reported by Solimannejad et al.16 Structures 1c, 2c, 3c/5c, and 6c of the cis-FA···N2O complex are stabilized by O−H···O, O−H···N, C−H···O, and C−H···N interactions, respectively. The interaction energies of the most stable structures 1c, 2c, and 3c are −9.5, −8.6, and −10.4 kJ mol−1 (after ZPVE and BSSE corrections) at the CCSD(T)-F12a/VDZ-F12 level of theory. The O−H···O, O−H···N, C−H···O bond lengths for 1c, 2c, and 3c are 2.04, 2.16, and 2.60 Å. Structure 7c is an energy minimum only at the M06-2X/aug-cc-pVTZ level and structures 8c and 9c are energy minima only at the MP2(full)/aug-cc-pVTZ level.


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Figure 1. Structures of the trans-FA···N2O complex optimized at the CCSD(T)-F12a/VDZ-F12 level of theory. The interaction energies (in kJ mol−1; after ZPVE and BSSE corrections) are given in parentheses. The geometrical parameters r, θ, and β represent the shortest distance and the angles between the complex units and are given in Table 1.


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Figure 2. Structures of the cis-FA···N2O complex optimized at the CCSD(T)-F12a/VDZ-F12 level of theory. The interaction energies (in kJ mol−1; after ZPVE and BSSE corrections) are given in parentheses. Structures 7c and 8c/9c are not energy minima at this level and are given at the M06-2X/aug-cc-pVTZ and MP2(full)/aug-cc-pVTZ levels (denoted as n.m.). The geometrical parameters r, θ, and β represent the shortest distance and the angles between the complex units and are given in Table 1.


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Table 1. Selected geometrical parameters of the optimized structures of the trans-FA···N2O and cis-FA···N2O complexes.a

a

trans-FA···N2O

r

θ

β

cis-FA···N2O

r

θ

β

1t

2.02

166.2

166.9

1c

2.04

171.0

123.2

2t

2.20

161.0

121.1

2c

2.16

178.8

170.6

3t

2.64

108.9

113.8

3c

2.60

110.4

113.5

4t

2.79

106.9

111.4

4c

3.02

94.0

84.7

5t

2.69

117.7

113.6

5c

2.96

94.1

85.7

6t

2.89

121.3

89.8

6c

2.74

108.8

111.8

7t

2.87

110.9

89.6

7c

2.10

135.4

106.4

8t

2.09

153.1

119.1

8c

2.94

98.7

90.1

9c

2.95

88.5

90.2

At the CCSD(T)-F12a/VDZ-F12 level except structures 7c and 8c/9c that are at the M06-

2X/aug-cc-pVTZ and MP2(full)/aug-cc-pVTZ levels. The shortest distance between the monomers (r) is in angstroms and the angles (θ and β) are in degrees.

The complexation-induced spectral shifts of the trans- and cis-FA complexes at CCSD(T)-F12a level of theory are given in Tables 2 and 3. The most significant shifts are predicted for the OH stretching and COH torsional modes of structures 1t, 2t, 8t, 1c, and 2c. The red shift of the OH stretching mode and the blue shift of the COH torsional mode are caused by the O−H···O and O−H···N interactions in structures 1t/1c/8t, and 2t/2c, respectively. For the structures with the C−H···O interactions (3t, 5t, 3c, and 5c), the largest shifts are obtained for the CH stretching mode. For the structures with the C−H···N interactions (4t, 6t, and 6c), the shifts are rather small. The vibrational frequencies for the monomers and the trans-FA···N2O and cisFA···N2O complexes at the M06-2X, wB97XD, MP2(full), and CCSD(T)-F12a levels of theory


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are given in Tables S3, S4, and S5 in the Supporting Information. It is seen that the different levels of theory predict similar spectral changes in the complexes. An exception occurs for the NN and NO stretching modes of N2O at the MP2(full) level of theory, which gives contradictory results compared to the DFT (M06-2X and wB97XD) and CCSD(T)-F12a levels (Tables S4 and S5).

Table 2. Calculated and experimental vibrational spectra (cm−1) of trans-FA and N2O monomers and the trans-FA···N2O complex.a monomers exptlb

calcd

tB

1t

2t

3t

shift exptl

4t

5t

6t

7t

8t

shift calcd

assignment

3550.5

3756.7

−82.3

−5.9

−77.3

−44.8

−1.7

0.3

−1.2

−0.1

−12.6

−43.5

OH stretch.

2953.0

3089.1

−84.7 −8.8

−6.2

−0.7

−1.1

14.3

6.3

13.4

0.6

8.0

−7.4

CH stretch.

2218.5

2281.9

14.5

10.7

14.6

14.4

9.6

7.4

8.4

7.0

3.1

2.8

NN stretch.

1767.4

1808.9

−11.5

−6.7

−14.9

−10.8

−10.1

−6.5

0.5

2.0

−3.3

−4.4

C=O stretch.

1381.0

1410.1

33.8

−2.6

9.6

7.2

−1.5

9.9

−1.3

−0.2

3.4

6.0

CH rock.

1282.9

1298.1

−20.6

−3.3

−18.9

22.2

−3.9

11.7

−0.4

6.2

3.9

−13.8

NO stretch.

1305.6c

1315.5

29.5e

6.1

34.3

18.6

4.4

10.6

−7.7

−6.4

2.3

14.9

CO−COH def.

1215.4c

a

tU

9.0

1103.6

1134.6

28.1

7.5

31.1

18.6

5.7

9.7

−8.2

−6.8

1.6

10.1

COH−CO def.

1038.2

1052.2

1.8

2.8

7.0

5.3

8.7

13.2

2.5

0.5

1.2

4.3

CH wagg.

635.4

670.5

70.1

6.1

80.5

49.2

6.3

14.9

−2.8

−2.6

12.4

56.7

COH tors.

629.2

630.7

18.4

2.3

20.8

13.8

3.6

10.0

−0.6

−0.8

5.6

12.9

OCO bend.

589.0

594.6 d

−9.5

−5.4

−8.6

7.6

−1.1

7.2

−0.7

3.0

5.1

−0.9

NNO bend.

−

594.6 d

−

−

−9.6

6.6

−5.9

2.8

−2.0

1.6

−1.3

−7.1

NNO bend.

At the CCSD(T)-F12a/VDZ-F12 level of theory. The shifts are the difference between the

complex and monomer frequencies. The shifts of the strongest absorptions are given in bold.

b


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trans-FA in site 2 according to Ref. 48. c Fermi resonance doublet between the CO−COH def. fundamental mode and the first overtone of OH torsional mode. dDoubly degenerated mode.

e

Calculated relatively to the average of the Fermi resonance values. Table 3. Calculated and experimental vibrational spectra (cm−1) of cis-FA and N2O monomers and the cis-FA···N2O complex.a monomers exptlb

calcd

3615.8

3819.9

cB

cU

1c

2c

shift exptl −20.0

3c

4c

5c

6c

shift calcd 1.9

−44.4

−26.3

−0.9

assignment 0.2

0.0

−1.2 OH stretch.

−30.4 2896.4

3004.3

−1.2

−6.3

−0.9

−2.5

19.6

13.3

16.0

7.7 CH stretch.

−17.5 2218.5

2281.9

9.8

10.8

7.8

22.2

10.0

7.6

7.4

8.3 NN stretch.

7.4 1807.0

1850.6

−6.4

−5.1

−5.0

−4.5

−9.8

−4.1

−7.7

−7.9

C=O stretch.

1391.9

1422.3

−

−2.2

2.6

2.3

0.7

−0.3

6.3

2.9

CH rock.

1282.9

1298.1

−6.2

−4.7

−16.9

22.6

−6.9

−2.2

−7.7

13.8

NO stretch.

1248.9

1293.0

16.1

9.3

38.7

34.9

3.8

−0.5

5.2

2.1

4.6

−1.7

CO−COH def.

−6.0

a

1107.8

1118.0

−

6.6

22.9

21.4

7.5

2.5

9.3

7.5

COH−CO def.

−

1030.7

−

−

6.8

6.1

9.5

3.8

4.1

7.3

CH wagg.

505.1

520.6

43.1

8.0

97.8

96.3

11.4

6.0

9.7

8.8

COH tors.

662.3

660.3

5.7

2.9

13.8

10.2

3.9

4.2

4.3

3.5

OCO bend.

589.0

594.6 c

−18.7

−5.6

−4.4

3.7

−1.9

−1.1

−3.0

5.8

NNO bend.

−

594.6 c

−11.3

1.2

−7.1

−3.6

−5.0

1.1

NNO bend.

−

−

At the CCSD(T)-F12a/VDZ-F12 level of theory. Complex 7c, 8c, and 9c are not energy minima

at this level. The shifts are the difference between the complex and monomer frequencies. The


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shifts of the strongest absorptions are given in bold.

b

cis-FA in site 2 according to Ref. 48.

c

Doubly degenerated mode.

The cis-to-trans torsional energy scans of the cis-FA monomer and cis-FA···N2O complex (structures 1c and 3c) are shown in Figure 3. The energies were calculated at the CCSD(T)-F12a/VDZ-F12 level by changing the torsional angle from the initial torsional angle of 180 degrees (cis-FA) to 0 degrees (trans-FA) with an increment of 10 degrees. In the torsional scan, the coordinates of all other atoms are frozen, which corresponds to the adiabatic approximation for fast tunneling.66 The energies are not corrected for ZPVE or BSSE. The performed simple torsional scans is known to slightly overestimate the barrier height.67 The cisto-trans barriers of 3109, 4115, 4085, 3257, and 3232 cm−1 were obtained for the cis-FA monomer and structures 1c, 2c, 3c, and 6c, respectively. For structures 4c and 5c, two different barriers exist for the torsional movement (i) toward the N2O unit and (ii) away from the N2O unit. The cis-to-trans barriers of structures 4c and 5c are 3088/3152 cm−1 and 3095/3292 cm−1 for the toward/away torsional movements. It is worth comparing the calculated properties of the FA···N2O complex and the isoelectronic FA···CO2 complex.33 The most stable structures of the trans-FA···CO2 and cisFA···CO2 complexes are formed by interaction of the OH group of FA with an oxygen atom of CO2. We have calculated these structures at the CCSD(T)-F12a/VDZ-F12 level used in the present work. The interaction energies of these structures (−15.7 and –10.4 kJ mol−1; ZPVE and BSSE corrected) are similar to those of structures 1t (trans-FA···N2O, −15.9 kJ mol−1) and 1c (cis-FA···N2O, −9.5 kJ mol−1). The O−H···O bond lengths are 2.04 and 2.07 Å for the strongest trans-FA···CO2 and cis-FA···CO2 complexes. These lengths are slightly larger than those of


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structures 1t (trans-FA···N2O; 2.02 Å) and 1c (cis-FA···N2O; 2.04 Å). The shifts predicted for the OH stretching/COH torsional modes of the trans-FA···CO2 and cis-FA···CO2 complexes are −56/+75 and −19/+81 cm−1, respectively. The shifts obtained for structures 1t (trans-FA···N2O) and 1c (cis-FA···N2O) are similar (−77/+81 and −44/+98 cm−1, respectively), although some difference occurs for the OH stretching mode of 1c.

5000

Relative energy (cm−1)

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cis-FA 1c 3c

4000 3000 2000 1000 0 0

30

60

90

120

150

180

Torsional angle (degree) Figure 3. CCSD(T)-F12a/VDZ-F12 energy scans for the cis-to-trans reorganization of FA monomer (red) and structures 1c (green) and 3c (black). The torsional angles of 0° and 180° correspond to the trans- and cis-FA conformations.

3.2 Experimental results. After deposition of an FA/N2O/Ar matrix, a set of absorptions is observed that do not belong to FA and N2O monomers or their multimers. These bands are consistent with the formation of the 1:1 complex between these two species. Figure 4a presents


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selected regions of the IR spectrum of an as-deposited FA/N2O/Ar matrix. For the O−H and C=O stretching modes, the trans-FA···N2O complex bands (marked as tB) are observed at 3468.2/3465.8 and 1755.9 cm−1, together with the known absorptions of the trans-FA monomer (3550 and 1767 cm−1)48 and trans-FA dimers (tt1: 1728 cm−1; tt2: 1748 cm−1 following Ref. 54). Other bands of tB are observed for the NN stretching (2218.5 cm−1), CO–COH deformation (1290.0 cm−1), NO stretching (1262.3 cm−1), COH–CO deformation (1131.7 cm−1), COH torsional (705.5 cm−1), and OCO bending (647.6 cm−1) modes. After annealing at 35 K, the monomer bands of trans-FA and N2O decrease in intensity and the tB bands increase (Figure 4b). The complexation-induced shifts of the OH stretching and torsional modes of tB from those of trans-FA monomer are significant. Based on this fact, we experimentally assign the tB bands to the trans-FA···N2O complex, in which N2O interacts with the OH group of trans-FA. The experimental shifts of the tB bands with respect to the monomer are given in Table 2. Vibrational excitation at 3466 cm−1 (the strongest component of the OH stretching mode of tB after annealing at 35 K) at 4.3 K consumes the tB bands and produces two new sets of bands, denoted here as tU (major channel) and cU (minor channel) (Figure 4c). The most characteristic bands of tU belong to the OH stretching (3544.6 cm−1), NN stretching (2229.2 cm−1), C=O stretching (1760.6 cm−1), NO stretching (1279.6 cm−1), COH−CO deformation (1111.1 cm−1), and COH torsional (641.5 cm−1) modes. The complexation-induced shifts of OH stretching and COH torsional modes of tU from those of trans-FA monomer are rather small. Based on this fact, we experimentally assign the tU bands to the trans-FA···N2O complex without an interaction between N2O and the OH group of trans-FA. The experimental shifts of the tU species are given in Table 2. The cU species is a minor product of the 3466 cm−1 pumping of tB and it is discussed below.


13


Excitation at 3544 cm−1 (the OH stretching frequency of tU) at 4.3 K consumes tU and produces new bands (denoted earlier as cU) at 3617.8, 2229.4, 1801.9, 1278.2, 1258.2/1247.2, and 513.2 cm−1 (Figure 4d). Based on the characteristic shifts, we experimentally assign the cU bands to the cis-FA···N2O complex without an interaction between N2O and the OH group of cis-FA. Excitation of the OH stretching mode of tU also produces a small amount of tB. The experimental shifts of the cU species are given in Table 3.

trans-FA 0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

trans-FA

0.4

tB

(a) tB (b) tU

0.2 cU (c)

cU

cU

tU

tB

tB cU

(d) x3 tU

0.0

tU 3600

3550

3500

1800

1750

−1

Wavenumber (cm )

Figure 4. FA···N2O complex in an argon matrix. FTIR spectra of (a) as deposited FA/N2O/Ar (1/1/1000) matrix; (b) after annealing at 35 K; (c) result of excitation at 3466 cm−1 at 4.3 K (difference spectrum); (d) result of excitation at 3544 cm−1 at 4.3 K (difference spectrum). The spectra were measured at 4.3 K.


14

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

15


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As pointed out above, vibrational excitation of tB at 4.3 K efficiently produces structure tU (Figure 5a). Upon annealing of the matrix at ~15 K, tU (prepared by vibrational excitation of tB at 4.3 K) converts back to tB at a subminute scale (Figure 5b). This annealing-induced conversion confirms that the tB and tU species are two structures of the trans-FA···N2O complex, tB (with the bonded OH group) being the more stable one. Figure 5c shows the result of vibrational excitation at 3466 cm−1 of a FA/N2O/Ar matrix performed at 15 K. Under these conditions, cU is mainly produced from tB. After stopping irradiation, cU decays mostly to tU (Figure 5d). As previously reported, selective vibrational excitation of the OH stretching mode of trans-FA at 3550 cm−1 efficiently produces cis-FA molecules (Figure 6a).46-50 If the matrix is annealed at 35 K during this excitation, a new set of bands denoted as cB in Figure 6b is produced (3595.9/3585.0, 2228.3/2225.9, 1800.5, 1276.7, 1265.0/1253.5/1242.9, and 548.2). In time, cB slowly decays to tU (Figure 6c). Based on the characteristic shifts, we assign the cB bands to the cis-FA···N2O complex, in which N2O interacts with the OH group of cis-FA. In the same way, the H-bonded complexes of cis-FA with CO2, N2, Xe, and H2O have been prepared.3335, 38

The experimental shifts of the cB species are given in Table 3.


16

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0.8

tU

tU cU

cU (a) 0.6

tB Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tB

tB 0.4

(b) cU

tU cU 0.2

(c) tB (d)

0.0

tU

tU cU

cU 3600

tB

3550

3500

1800

1750

−1

Wavenumber (cm )

Figure 5. FA···N2O complex in an argon matrix. Difference FTIR spectra showing the results of (a) excitation at 3466 cm−1 at 4.3 K of a FA/N2O/Ar (1/1/1000) matrix annealed at 35 K (similar to spectrum c in Figure 4); (b) annealing at 15 K of the previous matrix; (c) excitation at 3466 cm−1 at 15 K of a FA/N2O/Ar (1/3/1000) matrix annealed at 35 K after deposition; (d) waiting for ~50 min under Globar irradiation at 4.3 K for the previous matrix. The spectra were measured at 4.3 K.


17


cis-FA 0.8

cis-FA

trans-FA

1.0

trans-FA 0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

(a)

*

tB (b) x5

0.4

tB

(b) x5 0.2

.

(b) tU tU

0.0

(c) x5 cB

cB -0.2 3600

3550

3500

1800

1750

Wavenumber (cm−1)

Figure 6. FA···N2O complex in an argon matrix. FTIR spectra (a) after excitation at 3550 cm−1 at 4.3 K of a FA/N2O/Ar (1/3/1000) matrix; (b) after annealing at 35 K of a FA/N2O matrix upon excitation at 3550 cm−1; (c) the result of waiting for ~70 min under Globar irradiation at 4.3 K for the previous matrix (difference spectrum). The spectra were measured at 4.3 K. The bands marked with an asterisk and a dot are from the C=O stretching overtone of cis-FA monomer and the C=O stretching mode of a trans-cis-FA dimer (tc3 according to the notation of Ref. 54), respectively.


18

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The “dark” decay curves of two structures of the cis-FA···N2O complex and of cis-FA monomer are shown in Figure 7a. These data were obtained by measuring through a long-pass filter (1850 cm−1), which suppressed the effect of the spectrometer beam on the decay rate. The C=O stretching band of each species were integrated and the results were fitted with a single exponential function. The lifetimes of the cU and cB species and cis-FA monomer at 4.3 K are 18.7, 81.5, and 6.3 min, respectively. Our data for the decay of cis-FA monomer agrees with the literature.51 The decays of cU and cis-FA monomer were also measured at elevated temperatures (Figure 7b). The data show the low-temperature limit of the reaction, which is typical for H-atom tunneling.68 To recall, the activation of the reaction at higher temperatures (at ~20 K) is not an over-barrier process but explained in terms of matrix-reorganization energy.66 The temperature dependence of the cB was not measured because its band intensities are relatively weak and every data point required the preparation of a new matrix.


19


Relative concentration

1.0

(a) cB

0.5

cU

cis-FA 0.0 0

10

20

30

40

50

Time (min) (b)

-5

cis-FA ln k

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-6

cU

-7

0.05

0.10

0.15

0.20

0.25

1/T (K−1) Figure 7. (a) “Dark” decay of cis-FA, cU, and cB in an argon matrix at 4.3 K. The following bands were integrated: 1807.0 cm−1 (cis-FA, squares); 1801.9 cm−1 (cU, down-triangles); 1800.5 cm−1 (cB, circles). The lines are single exponential fits. (b) Arrhenius plots for the decay rate of the cis-FA and cU species. The lines guide the eye.


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4. DISCUSSION 4.1. Spectral assignment. The set of bands observed after deposition and increasing upon annealing (marked as tB in Figure 4) is assigned to structure 1t of the trans-FA···N2O complex (Table 2). The observed shifts of the OH stretching and COH torsional modes are consistent with the presence of the O−H···O and O−H···N interactions in 1t and 2t. However, structure 2t can be ruled out, because the calculated shift for the NO stretching mode (+22.2 cm−1) is of the opposite sign than the experimental shift (−20.6 cm−1) observed for tB. On the other hand, all calculated values obtained for 1t are in good agreement with the experimental shifts of tB (Table 2). The OH stretching, CH stretching, NN stretching, C=O stretching, NO stretching, COH−CO deformation, and COH torsional bands of this complex are shifted from the monomer bands, respectively, by −82.3/−84.7, –8.8, +14.5, −11.5, −20.6, +28.1 and +70.1 cm−1 in the experiment (tB) and by −77.3, –0.7, +14.6, −14.9, −18.9, +31.1 and +80.5 cm−1 in the calculations (1t). The CO−COH deformation mode of trans-FA monomer is split by Fermi resonance with the first overtone of the COH torsional mode whereas this mode of the tB complex is free of Fermi resonance. Therefore, an accurate comparison of the experimental and calculated shifts is not possible for this mode. However, the averaged shift from the Fermi components (+29.5 cm−1) agrees well with the calculated value (+34.3 cm−1). The experimental species tU is assigned to structure 3t of the trans-FA···N2O complex. The small shifts of the OH stretching and COH torsional modes show weak perturbation of the O−H bond as in structures 3t and 4t. Structure 4t can be ruled out based on the NO stretching shifts (theory: +11.7 cm−1; experiment: −3.3 cm−1). The OH stretching, NN stretching, C=O stretching, NO stretching, COH−CO deformation, and COH torsional modes of tU are shifted, respectively, by −5.9, +10.7, −6.7, −3.3, +7.5 and +6.1 cm−1, which is in a good agreement with


21


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Page 22 of 38

the calculations for structure 3t (−1.7, +9.6, −10.1, −3.9, +5.7 and +6.3 cm−1) (Table 2). In the CO−COH deformation region, the observation of two bands shifted from the Fermi resonance components of the monomer by +6.1 and +9.0 cm−1 suggest that CO−COH mode in the complex is also split. One can notice that the CH stretching mode is shifted in the experiment and in the calculations by −6.2 and +14.3 cm−1, respectively. This mismatch can be due to changes in interaction with the matrix (solvation) for this mode involved in the C–H···O interaction. It has been suggested that in this case, it is better to compare the complex in a matrix with the monomer in a vacuum.69-70 The CH stretching band of trans–FA in the gas phase is at 2942.8 cm−1,71 indicating a shift of +4 cm−1 which is closer to the calculated value. Furthermore, the geometry of the weakly bound complex can be modified by interactions with the surrounding argon atoms. Indeed, the calculations are performed for the species in a vacuum, whereas the experiment is made in a polarizable matrix. For the experimental species cU, the OH stretching, NN stretching, C=O stretching, NO stretching, COH–CO deformation, and COH torsional modes are shifted by +1.9, +10.8, −5.1, −4.7, +9.3/−1.7 and +8.0 cm−1, respectively. Based on the calculated shifts, structures 3c, 4c, and 5c are possible for this species (Table 3). However, the IR-induced tU → cU and tunnelinginduced cU → tU processes are dominant, the tU complex having structure 3t. From the geometrical considerations, structure 3c is the most suitable candidate for the experimental complex cU; therefore, we assign it accordingly. The mismatch between the experimental and calculated shifts of the CH stretching mode can be explained by the same rationale used above for tU. It is interesting to notice that the OH stretching mode of cU is slightly blue-shifted. To our knowledge, it is the first example of an experimental blue shift of this mode of FA upon complexation.


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The cB experimental species is assigned to structure 1c of the cis-FA···N2O complex. The shifts observed for the OH stretching and COH torsional modes suggest the O−H···O or O−H···N interactions as in 1c or 2c. However, structure 2c can be ruled out based on the NO stretching mode (Table 3). The experimental shifts of the OH stretching, NN stretching, C=O stretching, NO stretching, COH−CO stretching, and COH torsional modes of cB (−20.0/−30.4, +9.8/+7.4, −6.4, −6.2, +16.1/+4.6/−6.0 and +43.1 cm−1) are in a reasonable agreement with the calculations for 1c (−44.4, +7.8, −5.0, −16.9, +38.7 and +97.8 cm−1) (Table 3). The splitting of the OH stretching, CH stretching, NN stretching, and CO−COH deformation bands is presumably due to matrix site effects. As previously, the agreement in less satisfactory for the CH stretching mode. 4.2. Tunneling reaction. Figure 7a shows that the “dark” decay of species cB (lifetime of 81.5 min) and cU (lifetime of 18.7 min) is substantially slower than that of cis-FA monomer (lifetime of 6.3 min). The decay of cis-FA monomer as well as of a number of other complexes of cis-FA occurs via H-atom tunneling through the torsional barrier, as discussed elsewhere.49-50, 67

We suggest that the decay of the cB and cU species also occurs by H-atom tunneling. Several

factors can affect the tunneling rates in low-temperature rare-gas matrices. The most important factor controlling the decay of cis-carboxylic acids is the cis-to-trans barrier.49-52 The cis-to-trans torsional scans of cis-FA monomer and structures 1c and 3c of the cis-FA···N2O complex show barrier heights of 3109, 4115, 3257 cm−1 for these species, respectively (Figure 3). Thus, the calculated barriers are in full agreement with the order of the experimental decay rates. One experimental fact is difficult to fully understand. The cis-to-trans barrier for the cisFA···CO2 complex (4089 cm−1 at the CCSD(T)F12a/VDZ-F12 level) is close to that of structure 1c of the cis-FA···N2O complex with a similar arrangement of the complex units (4115 cm−1).


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Despite the similar stabilization barriers, the lifetime of cis-FA···CO2 complex was reported to be 13.3 h at 10 K,33 which is an order of magnitude longer than that of structure 1c of the cisFA···N2O complex. We can suggest three possible reasons for this mismatch. First, it has been found that the vibrational levels behind the torsional barriers are important for the tunneling process due to energy conservation.51, 72 The energy mismatch between the energy of the initial configuration should not differ much from the energy of the accepting level (in the scale of the Debye frequency of the matrix). The interaction energies in these two complexes somewhat differ from each other, which changes the energy mismatches. Second, the local matrix morphology can be different for these two complexes. As we have recently discussed, the true reaction coordinate is not exclusively the hydrogen motion but rather the collective coordinate involving the reorganization of matrix molecules.73 Finally, as we pointed out above, the calculations are performed for the species in a vacuum, whereas the experiment is made in a polarizable matrix. The geometry of the weakly bound complexes can be modified by interactions with the surrounding argon atoms and the matrix solvation effect can differ between these two complexes. Despite these possibilities, the relatively low stability of the 1c structure of the cis-FA···N2O complex is surprising, in our opinion. Explanation of this observation is beyond the scope of the present work.


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infrared 3c

*

annealing

3t infrared

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tunneling

* 1c

1t

Figure 8. Main processes observed in the present work. For the infrared excitation of 1t and 3t, the processes marked with an asterisk have a relatively low efficiency compared to the second channels.

5. CONCLUSIONS In the present work, we have studied the interaction of formic acid (FA, HCOOH) with nitrous oxide (N2O). FA and N2O participate in atmospheric chemistry, where non-covalent interactions play an important role. These two molecules appear in the list of interstellar species. This fact is also a motivation because the detection of weak complexes in interstellar space is a challenge and the laboratory experiments on relevant species are needed.74 The studied complex has a number of interesting fundamental features discussed below. The geometries, interaction energies, and vibrational spectra of the FA···N2O complex were calculated at the DFT (M06-2X


25


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and wB97XD), MP2(full), and CCSD(T)-F12a levels of theory. Eight structures of the transFA···N2O complex and nine structures of the cis-FA···N2O complex were obtained at these levels (Figures 1 and 2). Two structures of the trans-FA···N2O (1t and 3t) complex and two structures of the cis-FA···N2O (1c and 3c) complex were identified by infrared spectroscopy in an argon matrix. The calculated and experimental spectra of these species are in good agreement (Tables 2 and 3). The observed processes are summarized in Figure 8. Structure 1t of the trans-FA···N2O complex is observed after deposition of a FA/N2O/Ar matrix and its amount increases upon annealing. Excitation of the OH stretching mode of 1t at 4.3 K mainly leads to structure 3t of the trans-FA···N2O complex, i.e. the O−H···O interaction in 1t reorganizes to the C−H···O interaction in 3t. This process probably occurs by a flip of the trans-FA unit. A minor formation of structure 3c with the C−H···O interaction is observed at this pumping, which means the transto-cis conversion with a subsequent reorganization of the complex. Excitation of the OH stretching mode of 3t mainly leads to structure 3c of the cis-FA···N2O complex, i.e. the C−H···O interaction retains and the free OH group reorganizes to the cis configuration. Structure 3c is spectroscopically unusual because it shows a blue shift of the free OH stretching mode; however, we have no strong theoretical support to this experimental fact. Structure 3t of the trans-FA···N2O complex reorganizes to the more stable structure 1t upon annealing at 15 K at sub-minute scale. This decay indicates a very low stabilization barrier of 3t, which is unusual for these kinds of complexes. If excitation of the OH stretching mode of 1t is performed at 15 K, mainly structure 3c of the cis-FA···N2O complex is formed. This temperature-induced switching of the IR-pumping product is a remarkable observation. Structure 3c decays at ≤10 K exclusively to structure 3t of the trans-FA···N2O complex even in the dark at


26

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a scale of about 20 min presumably by H-atom tunneling. When the decay of 3c is monitored at 15 K and above, exclusively the formation of structure 1t is observed. Of course, it is not a true temperature-induced switching of the tunneling decay product, because the 3c → 3t → 1t cascade process presumably occurs: the first step operates by H-atom tunneling and the second one by thermal relaxation. Annealing of the matrices containing both cis-FA and N2O produces the cis-FA···N2O complex having structure 1c with the O−H···O interaction. This complex decays at 4.3 K in the dark to structure 3t of the trans-FA···N2O complex by H-atom tunneling. In this process, the O−H···O interaction in 1c breaks and the C−H···O interaction in 3t forms. The remaining question is why the lifetime of 1c is much shorter than that of the cis-FA···CO2 complex with the similar stabilization barrier and the structural motif. No 3c → 1c process was observed upon annealing at 20 K. This observation shows that the 3c → 1c stabilization barrier is higher that the 3t → 1t stabilization barrier.

ASSOCIATED CONTENT Supporting Information. Calculated interaction energies of the trans-FA···N2O and cisFA···N2O complexes (Table S1), selected geometrical parameters of the optimized transFA···N2O and cis-FA···N2O complexes (Table S2), calculated vibrational frequencies and infrared intensities of trans-FA, cis-FA, and N2O monomers (Table S3), calculated vibrational frequencies and infrared intensities of the trans-FA···N2O complex (Table S4), calculated vibrational frequencies and infrared intensities of the cis-FA···N2O complex (Table S5), and full references.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by the Academy of Finland through the Project KUMURA (No. 1277993). The CSC-IT Center for Science is thanked for computational resources.


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Complex between Formic Acid and Nitrous Oxide: A Matrix-Isolation

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