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Ab Initio Study Predicts That Enigmatic Isonitrosyl Fluoride Should Be Stable at Low Temperatures Yet Unnoticeable Due to Its Photoreactivity Raulia R. Syrlybaeva, and Marat R. Talipov J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b12130 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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The Journal of Physical Chemistry

Ab Initio Study Predicts That Enigmatic Isonitrosyl Fluoride Should Be Stable at Low Temperatures Yet Unnoticeable Due to Its Photoreactivity Raulia R. Syrlybaeva, Marat R. Talipov* Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico 88003, United States.

ABSTRACT:

Isonitrosyl fluoride F—ON remains an undetected molecule despite multiple attempts

to generate it and successful identification of other isonitrosyl halides (X—ONs) via phototransformations of corresponding X-NOs. We investigated this problem using ab initio methods and found no evidence of instability of F—ON at low temperatures of 8-10 K. Instead, experimental observation of F—ON is likely challenged by the (1) different nature of photoexcitation of F-NO and its lower quantum yield than in other X-NOs, and (2) presence of a bright charge-transfer transition in the F—ON spectrum that likely overlaps with the weak band of F-NO used for photoexcitation. Formation of F—ON via symmetry-prohibited photoexcitation of F-NO is followed by its immediate photodecomposition to the charge-transfer excited state and its conversion to F-NO upon de-excitation. Thus, F—ON should be readily observable using non-photochemistry methods such as microwave spectroscopy.

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INTRODUCTION

Nitrosyls X–NO (where X is a monatomic or diatomic substituent) have been extensively studied for several decades1–23 due to their unusual structure,2–4 importance in atmospheric chemistry,5–7 and possible practical application.8 No less interesting are their isonitrosyl counterparts X— ON,3,24–37 in which X forms a bond with the oxygen atom. Such molecules are stabilized by an unusual attractive interaction of unpaired electrons on N and X via the lone pair on the central oxygen atom.24,25 Several isonitrosyl compounds have been theoretically predicted29,35 and successfully generated, including HON,26 Br—ON,27 Cl—ON, 27 NC—ON28 and HO—ON.30 At the same time, the experimental observation of F—ON remains an unresolved problem despite numerous attempts.27,31,32 Spectral signatures of F—ON (i.e. IR bands at 1886.6, 734.9, and 492.2 cm-1) were claimed to be observed for the first time in 1974 via a co-deposition and photolysis of dilute argon and nitrogen matrices of fluorine and nitric oxide.31,32 However, subsequent studies revealed that these energies did not match the calculated vibrational frequencies33–35 and correspond to at least two different species,27 thus suggesting that discovery of F—ON must be revised. Mayer et al27 further established that nitrosyl fluoride could not be transformed into F—ON by lowtemperature (8 K) ultraviolet light-induced photoisomerization despite that similar attempts were successful for the generation of Cl—ON, Br—ON, H-ON and NC—ON. Interestingly, theoretical studies of F—ON demonstrated that lack of observation of F—ON cannot be attributed to the weakness of the F—O bond, as it forms a relatively deep local minimum on the potential energy surface of F(NO)35,36. Furthermore, the CCSD(T) calculations35 showed that the activation energy of the F—ON ⟶ F-NO transformation of 25.5 kJ/mol is sufficiently high to make F—ON a stable molecule at least at low temperatures.


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The inconsistency between the expected thermal stability and experimental undetectability of F-ON was tentatively attributed to its rapid interconversion into F-NO due to either tunnel effect or contribution of low-lying electronic excited states.27 In this study, we inspected both mechanisms by using ab initio calculations and theoretical modeling and found that neither mechanism could explain the seeming instability of F—ON at 8 K. At the same time, analysis of the excited electronic states of F—ON revealed the presence of a bright transition that may overlap with a weak symmetry-prohibited band of F-NO at ~300 nm used for photoexcitation. This excited state of F—ON has a prominent charge-transfer character, and it readily undergoes transformation into F-NO. Thus, the results of this study, described below, indicate that lack of observation of F—ON is likely related to its photolability and spectral overlap with F-NO. THEORETICAL METHODS

Full geometry optimizations of F-NO, F—ON, and the corresponding transition state were performed at the CCSD(T)-F12/aug-cc-pVTZ, MRCI(18;12)/aug-cc-pVTZ (8 ref. states) and MRCI-F12(18;12)/aug-cc-pVTZ (8 ref. states) levels of theory with the MOLPRO38 package, at the CCSD(T)/aug-cc-pVTZ level of theory with ORCA 4.039 package and at the UCCSD(T)/augcc-pVTZ level of theory with the Gaussian 1640 package. Full geometry optimizations of Br(NO), Cl(NO) were performed at the CCSD(T)/aug-cc-pVTZ level of theory with the Gaussian 16 package and at the UCCSD(T)/aug-cc-pVTZ level of theory with the ORCA package. Equilibrium structures of the excited-state charge-transfer complexes of FNO and Cl-NO were found using MRCI(18;12)/aug-cc-pVTZ method (5 ref. states) with the MOLPRO package. Potential energy surface (PES) calculations for F(NO) system were performed for R(NO) = 1.10 Å using the CCSD(T)-F12, CASSCF(18;12) (8 ref. states) and MRCI-F12 (18;12) (8 ref. states) methods with the aug-cc-pVTZ basis set, using MOLPRO package.


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Potential energy curves (PECs) for F-NO system dissociation along the F-N bond were performed using ORCA and Molpro packages at the MRCI(18;12)/aug-cc-pVTZ level of theory. The equilibrium geometry parameters of FNO obtained from the MRCI-F12(18;12)/aug-ccpVTZ method (R(NO)=1.134 Å and ÐFNO=110.0°) were used in PECs calculations with ORCA software for first eight singlet states. In PECs calculations using Molpro package (first eight singlet and first eight triplet states), the geometry parameters obtained from the UCCSD(T)/augcc-pVTZ calculation were applied (R(NO)=1.1375 Å and ÐFNO=110.0°). PECs for F-ON system dissociation along the F-O bond were performed at the MRCI(18;12)/aug-cc-pVTZ level of theory using ORCA package. The equilibrium geometry parameters of FON obtained from the MRCI-F12(18;12)/aug-cc-pVTZ method at R(NO)=1.112 Å and ÐFNO=115.8° were used in PEC calculations (first eight singlet and first eight triplet states). Calculations of lowest-energy path of the F-NO/F—ON isomerization and of energies of the excited states along this path were performed at the CASPT2(18;12)-F12/aug-cc-pVTZ level of theory using MOLPRO package. Harmonic vibrational frequency calculations were carried out at the CCSD(T)/aug-cc-pVTZ level of theory with the ORCA package, at the UCCSD(T)/aug-cc-pVTZ level of theory with the Gaussian 16 package, and at the MRCI(18;12)/aug-cc-pVTZ level of theory using in-house software. The electronic excitation calculations of nitrosyl/isonitrosyl halides were performed using ORCA package at the MRCI(18;12)/aug-cc-pVTZ level of theory, the geometries were taken from UCCSD(T)/aug-cc-pVTZ geometries. RESULTS AND DISCUSSION

To explore thermally allowed mechanisms of interconversion of F—ON into F-NO, we first analyzed the potential energy surface (PES) of the ground 11A' electronic state of F(NO) (Figure


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1). This analysis, undertaken at the CASSCF/aug-cc-pVTZ level of theory with full valence active space consisting of 18 electrons occupying 12 valence orbitals (see the SI for the details), revealed two minima (i.e. F–NO and F—ON) without a clear saddle point between them (Figure 1A). In contrast, similar calculations at the MRCI level (Figure 1B), which is capable of accounting for dynamic electron correlation, showed the presence of the expected transition state. The PES, similar to that in Figure 1B, was also reproduced by the coupled cluster calculations (Figure S1 in the SI). Absence of other intermediates and alternative paths indicates that F—ON ® F–NO transformation on the ground state PES likely occurs only through the above-mentioned transition state or the formation of the F/NO radical pair. Next, we computed the equilibrium structures of F—ON, F–NO, and the transition state of their interconversion (TS) at the MRCI-F12(18;12)/aug-cc-pVTZ level of theory (Figure 1B) and found that the obtained geometries were in a good agreement with the early theoretical report by T. Lee,14,35 performed at the CCSD(T)/TZ2P level (Tables 1-3). Table 1. Comparison of the structural parameters of F-NO, obtained by using various computational approaches Level of Theory CCSD(T)/TZ2P12 CCSD(T)-F12/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ UCCSD(T)/aug-cc-pVTZ MRCI(18;12)/aug-cc-pVTZ MRCI-F12(18;12)/aug-cc-pVTZ

RFN, Å 1.536 1.509 1.520 1.520 1.498 1.503

RNO, Å 1.137 1.134 1.137 1.137 1.139 1.134

ÐFNO, º 110.0 110.0 110.0 110.0 110.1 110.0

Table 2. Comparison of the structural parameters of F—ON, obtained by using various computational approaches Level of Theory CCSD(T)/TZ2P [35] CCSD(T)-F12/aug-cc-pVTZ

RFO, Å 1.802 1.796

RNO, Å 1.111 1.103

ÐFON, º 114.9 114.2


5


CCSD(T)/aug-cc-pVTZ UCCSD(T)/aug-cc-pVTZ MRCI-F12(18;12)/aug-cc-pVTZ MRCI(18;12)/aug-cc-pVTZ

1.800 1.774 1.730 1.765

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1.108 1.099 1.112 1.116

114.4 113.6 115.8 116.2

Table 3. Comparison of the structural parameters of TS (bond lengths are in Å), obtained by using various computational methods Level of Theory CCSD(T)/TZ2P35 CCSD(T)-F12/aug-cc-pVTZ CCSD(T)/aug-cc-pVTZ UCCSD(T)/aug-cc-pVTZ MRCI(18;12)/aug-cc-pVTZ

RFO 2.004 1.994 2.001 1.953 2.013

RNO 1.096 1.087 1.092 1.078 1.113

RNF 2.204 2.202 2.206 2.237 2.189

It is noted that the 2.0 Å separation between the fluorine atom and the NO moiety in the transition state is less than the corresponding Van der Waals distance (~3.0 Å),41 thus hinting at a significant energetic stabilization of the TS. In agreement with this expectation, the relative energy of the TS was found to be 62 kJ/mol lower than that of the separated F/NO radicals. The activation energy of the F—ON ® F–NO isomerization (42 kJ/mol, see Table S1 in the SI) and vibrational frequencies of F–NO and the TS, obtained at the MRCI(18;12)/aug-cc-pVTZ level of theory, were used for the evaluation of the classical rate constant at 8 K (temperature was chosen to reproduce the experimental conditions). We used Rice-Ramsperger-KasselMarkus (RRKM) theory according to eq. 1:42 %& 𝑘"#$ =

‡ () * +,-

ℎ +,-

exp(

345

() *

)

(eq 1)

where kB and h are Boltzmann and Planck constants, T is temperature, Q‡vr and Qvr are the vibrational/rotational partition functions for the TS and F—ON, respectively, and Ea is the ZPEcorrected electronic energy of activation. Expectedly, the classical rate constant of isomerization was found to be negligibly small, ~7.1 × 10-246 s-1, due to the large ratio between the saddle point


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energy (V1 = 3480 cm-1) and thermal energy at 8 K (kBT = 5.6 cm-1). Thus, F—ON should be a stable compound with respect to classical isomerization under low-temperature conditions. A

CASSCF(18;12)/aug-cc-pVTZ 3

2

yF, Å 1

0.0 kJ/mol

183 kJ/mol

N

0 −2

B

−1

O 0

xF, Å

1

2

F

MRCI-F12(18;12)/aug-cc-pVTZ 2.0

N

N

O

O

N

O

1.113 Å

1.5

F 1.498 Å 110.1º

F

2.013 Å 83.6º

yF, Å

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


1.765 Å −1

0

xF, Å

1

1.139 Å

F

116.2º

N

O

1.116 Å

Figure 1. Partial potential energy surface (PES) of FNO (X), calculated at the CASSCF(18;12) (A) and MRCI-F12(18;12)+Q (B) levels at R(NO) = 1.10 Å using aug-cc-pVTZ basis set. Both PESs were constructed by varying Cartesian coordinates (XF, YF) of F atom with fixed coordinates of N (-0.55 Å, 0) and O (+0.55 Å, 0) atoms. Panel B also shows equilibrium geometries of F–NO, F—ON, and the transition state of their interconversion, obtained at the MRCI(18;12)/aug-cc-pVTZ level. Inset in panel B schematically represents the reaction coordinate as a rotational displacement of the NO moiety. Furthermore, we inspected whether the F—ON/F-NO isomerization rates could be elevated by the tunnel effect. Such a possibility seems particularly plausible for the isomerization of isonitrosyl compounds, since such isomerization does not involve the transfer of the fluorine atom and requires only a relatively small rotational displacement of the NO moiety (see the inset


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in Figure 1B),27 associated with a reduced effective mass of ~15 amu. The tunnel effect was evaluated using eq. 2: 𝑘"#$ =

∞ 7 𝑘(𝐸) 𝑁(𝐸)𝑒 34/() * 𝑑𝐸 +,- 9

(eq 2)

where N(E) is the density of states, k(E) is the microscopic rate constant, and energy is referenced to the ground vibrational state energy of F—ON. Vibrational states were directly enumerated using the computationally efficient recursive Beyer-Swinehart algorithm43 with 1 cm-1-wide bins. The rotational partition functions (i.e. 43.9 for F-NO and 113.1 for F—ON) and the density of the rotational states were calculated using the standard three-dimensional rotor approximation (see eqs A1.8 and A1.14 in ref. 42). Since the vibrational partition functions for both the TS and F—ON were nearly identical to one [(Q - 1) < 10-10 at 8 K], k(E) was evaluated according to eq. 3 rather than the RRKM approximation:44 𝑘(𝐸) =

?@ AB

𝑃(𝐸)

(eq 3)

where w1 is the frequency of the bending mode in F—ON (Table S2 in the SI), and P(E) is the tunneling probability, evaluated using the asymmetric Eckart potential (see the SI for details).45,46 The contribution from the tunnel effect (eqs 2 and 3), was found to increase the rate constant from 7.1 × 10-246 s-1 to 9.7 × 10-5 s -1 owing to the involvement of the ground vibrational and lowenergy (E < 50 cm-1) rotational states (Figure S3). However, even such a dramatic acceleration is clearly not sufficient for making F—ON an unstable molecule at low temperatures. It was also hypothesized that the F—ON ® F–NO isomerization could be facilitated by the low-energy states of the symmetry/multiplicity different from that of the ground state.27 To address this possibility, we calculated the energies of the 1A", 3A' or 3A" states along the F—ON isomerization and dissociation profiles (Figures S2, S4-S5 in the SI) and found no evidence of crossing of the ground and excited states. Moreover, the lowest-energy excited state was found to


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be at least 95 kJ/mol higher in energy than the ground state along the isomerization path, thereby excluding the possibility that excited states might be involved in thermal transformations of F— ON. The results presented above show that F—ON should be a thermally stable molecule at low temperatures. At the same time, it is interesting to note that the reported attempts to synthesize F—ON were based on photochemical excitation of F-NO.27,31,32 For example, Mayer et al attempted to generate F—ON by the light-induced (185-700 nm) photochemical isomerization of F-NO at low temperature (8 K).27 This approach did not lead to the formation of the expected spectral signatures of F—ON , although it worked successfully for several other isonitrosyls, i.e. HON, Br—ON, Cl—ON, NC—ON.26–28 Accordingly, we investigated whether the lack of observation of F—ON could be attributed to some specific features of its photochemical transformations. Figure 2 shows the experimental electronic spectra of Br-NO + Br—ON, Cl-NO + Cl—ON, and F-NO, together with the computed electronic excitation energies. The remarkable agreement between the calculated and experimentally observed spectra clearly demonstrated the reliability of the computational approach and enabled us to interpret the features of nitrosyl/isonitrosyl spectra using insights from the electronic structure calculations. It is noted that successful generation of Br—ON and Cl—ON required irradiation of Br-NO and Cl-NO at the wavelengths corresponding to their low-energy bright transitions (248 and 193 nm, respectively). Our calculations show that these transitions lead to the 41A' state in Br-NO and Cl-NO, with a dipole moment of ~8 D greater that of the ground state (~2 D). Based on a large value of the dipole moment, we conclude that generation of Br—ON and Cl—ON was intermediated by the formation of the charge-transfer electronic state, as suggested in refs 20-23.


9


1.2

Experimental 27 200 220 193

1 0.8

440

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

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0.6

248

0.4

~400

0.2 0

Calculated 130 184 203

-0.2 100

150

200

313 386

265 250

300

350

400

448 450

500

550

600

Wavelength, nm

Figure 2. Optical spectra of Br-NO (solid blue line), Br—ON (dotted blue line), Cl-NO (solid green line), Cl—ON (dotted green line).27 Green and blue dots on optical spectra denote the irradiation wavelengths at which the photoisomerization of corresponding nitrosyls to isonitrosyl was conducted, the black dot is the position of the observed low-energy band of F-NO.27 Vertical sticks denote the absorption maxima for the same compounds, calculated at the MRCI(18;12)+Q/aug-cc-pVTZ level of theory using the atomic coordinates optimized at the CCSD(T)/aug-cc-pVTZ level (Tables S3-S5 in the SI). The above-mentioned transition in Cl-NO showed a prominent blue shift from the corresponding transition in Br-NO thus suggesting a similar blue shift for the charge-transfer transition in the absorption spectrum of F-NO. Indeed, the MRCI calculations showed a chargetransfer electronic excitation of F-NO at 130 nm, which corresponds to the far ultraviolet region of the spectrum. Photoexcitation of F-NO at 130 nm is problematical due to the large density of electronic states mixed with the charge-transfer state and low fosc value of 0.02. For comparison, the charge-transfer photoexcitations of Cl-NO and Br-NO have fosc values of 0.49 and 0.47, respectively (Table 4). Table 4. Wavelengths (l, nm) and oscillator strengths (fosc) for the electronic excitations of nitrosyl/isonitrosyl halides [MRCI(18;12)//UCCSD(T)/aug-cc-pVTZ]


10

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State 11A'' 21A'' 21A' 31A' 31A'' 41A' 41A''

Br-NO fosc l 538.3 0.000 399.8 0.000 377.8 0.001 296.9 0.002 291.1 0.002 203.5 0.471 155.7 0.050

Br—ON fosc l 2269.3 0.000 1600.4 0.000 1553.7 0.000 1419.5 0.000 1380.8 0.000 448.8 0.206 158.8 0.002

Cl-NO fosc l 447.1 0.000 324.2 0.000 310.7 0.001 242.8 0.001 237.0 0.001 184.5 0.492 163.6 0.000

Cl—ON fosc l 1378.2 0.000 941.8 0.000 930.1 0.000 829.5 0.000 815.9 0.000 386.3 0.296 154.0 0.000

F-NO fosc l 307.5 0.002 211.0 0.000 197.3 0.004 148.6 0.001 146.1 0.000 130.2 0.020 135.8 0.004

F—ON fosc l 573.2 0.000 411.8 0.000 411.2 0.000 361.3 0.001 358.0 0.000 265.0 0.342 131.8 0.001

The experimental attempts to generate F—ON were focused on the photoexcitation of F-NO at 185-700 nm.27 Specifically, F-NO was excited to a weak, symmetry-prohibited absorption band at ~300 nm, which, as our calculations demonstrate in agreement with the literature data,11–13,36 corresponds to the dissociative S1 ⟵ S0 transition that is followed by the formation of the F/NO radical pair. This transition also has low value of the oscillator strength (fosc = 0.002), and therefore photochemical formation of F—ON by photoexcitation of F-NO at 300 nm is expected to have low quantum yield. The observed blue shift of the bright absorption band in the series of nitrosyl bromidechloride-fluoride suggests that a similar blue shift might also be found in their isonitrosyl counterparts. In agreement with this expectation, the experimental optical spectra of Br—ON and Cl—ON showed bright absorption bands centered at 440 and 400 nm, respectively (Figure 2).27 Our MRCI calculations reproduced these values with remarkable accuracy (Figure 2) and also demonstrated large dipole moments in the produced excited states (12.1 and 10.5 D), which were indicative of their charge-transfer character. Similar calculations of the excited states of F—ON revealed that F—ON should also have a bright transition (fosc = 0.34) to a charge-transfer state (Figure 2). Surprisingly, the characteristic lmax value of this transition (i.e. 240-276 nm according to different estimates, see Table 4) closely matches the band of F-NO at 313 nm targeted experimentally (for example, the Mayer


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group tested the 185-700 nm range during their attempts to obtain F—ON and observed that the bands of FNO diminished and the absorption of NO˙ became stronger at ~313 nm).27 It is noteworthy that the experimental absorption spectra of Cl—ON and Br—ON show a broad lowenergy shoulder extended by ~100 nm from the band center (Figure 2), and therefore F—ON could also be expected to have a similar shoulder extending to ~350 nm, which likely overlaps with the F-NO band at 313 nm used for photoexcitation. Overlap of the spectral features of F-NO and F—ON motivated us to explore the PES of the charge-transfer state. Interestingly, dissociation curves for the F–N and F–O bonds in the chargetransfer (i.e. 41A') states of nitrosyl and isonitrosyl fluoride, respectively (Figure 3), showed a prominent minimum, thus hinting at the presence of a global minimum on the charge-transfer PES. Indeed, geometry optimization of the charge-transfer state of F-NO led to the equilibrium geometry shown in Figure 4A. The structure of the charge-transfer state was found to be somewhat similar to the TS for the F-NO/F—ON isomerization on the ground electronic state, with the F—O bond length being 0.71 and 0.45 Å longer than in the ground-state F—ON and the TS, respectively. A similar charge-transfer complex in 41A' state was also found for the case of chlorine (Figure 4A), thus suggesting that the existence of nitrosyl/halide charge-transfer complexes is a common feature of isonitrosyl halides.


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A

B 9.0

10.0

NO(A2S +), F2P

9.0

8.0

41A'

8.0

NO(A2S +), F2P

7.0

7.0

5.0 4.0

NO(X), F2P

3.0

Energy, eV

6.0

6.0

Energy, eV

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


4.0 3.0

2.0

2.0

1.0

1.0

NO(X), F2P 1X

1X

0.0

41A'

5.0

0.0

-1.0

-1.0 1.3

2.3

3.3

1.5

2.0

R(N-F), Å

2.5

3.0

3.5

4.0

R(O-F), Å

Figure 3. Singlet dissociation curves for the F-N bond of F-NO (A) and F—O bond of F—ON (B) (also see Figures S2 and S3 in the SI) [MRCI(18,12)+Q/aug-cc-pVTZ]. Solid and dashed lines denote electronic states with A' and A'' symmetry, respectively. A

N

N 70.4

0

2.475 Å

71.20

1.069 Å F

84.20

O

B

3.045 Å

1.079 Å

O

2.343 Å

Cl

88.10 2.884 Å

N

N -0.279 0.041

-0.312

0.00004

0.020

-0.00003

F

O

Cl

O

Figure 4. Equilibrium structures of the excited-state charge-transfer complexes of F-NO and ClNO (A), and the corresponding gradients (in atomic units) on the ground electronic state (B) [MRCI(18;12)/aug-cc-pVTZ]. The obtained equilibrium geometry of the excited-state charge-transfer complex (Figure 4A, left) could not be readily attributed to either F-NO or F—ON domain on the ground-state PES. To identify the fate of F/NO system after de-excitation of the charge-transfer complex, we calculated forces acting on the fluorine atom in the ground electronic state but at the equilibrium


13


geometry of the corresponding nitrosyl/fluoride complex (Figure 4B). For comparison, similar calculations were performed for the nitrosyl/chloride charge transfer complex. From these calculations, the energy gradient was found to be oriented toward the formation of Cl—ON in the case of chlorine but toward the formation of F-NO in case of fluorine. The findings presented above suggest that stationary photoexcitation of F-NO at ~300 nm creates a photochemical equilibrium between F-NO and F—ON (Figure 5A). The forward reaction, i.e. isomerization of F-NO into F—ON (reactions 1 and 2 in Figure 5A), involves the symmetry-prohibited electronic excitation of F-NO and therefore is expected to have a low quantum yield. On the other hand, the reverse reaction (reactions 3-5 in Figure 5A) is expected to have a high quantum yield due to the bright charge-transfer band of F—ON, which facilitates the formation of the charge-transfer complex and its conversion into F-NO. Thus, the equilibrium constant of the F-NO/F—ON photoisomerization is expected to be significantly shifted toward the formation of F-NO. B

10 (F-NO)CT at 130 nm

F-NO

5

(F—ON)CT

1 2’

1:

5

5

CT

2’: (F+NO)diss hν 3: F—ON 4: (F—ON)CT CT CT

Cl—ON

(Cl-NO)CT 2

1

CT 3’

3

F—ON 0

hν

Cl-NO

(Cl+NO)diss

F-NO F-NO

10

3 5’

2: (F+NO)diss

5: 5’:

4

(F+NO)diss 2

0

F—ON

E, eV

A E, eV

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(F+NO)diss F + NO

F—ON

F + NO (F—ON)CT CT

F-NO

Cl—ON

Cl-NO

1: Cl-NO 2: (Cl-NO)CT 3: CT 3’: CT

hν

(Cl-NO)CT CT Cl-NO Cl—ON

F-NO F—ON

Figure 5. Schematic depiction of unimolecular photoreactions of F-NO (A) and Cl—NO (B). (X-NO/ON)CT and (F+NO)diss denote charge-transfer and F/NO dissociative states at the


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corresponding ground-state equilibrium geometries, while CT denotes equilibrium geometries of the charge-transfer complexes. This conclusion could be further justified by considering the photoisomerization of F-NO into F—ON and F—ON into F-NO in Figure 5A as separate processes (reactions R1 and R2): F-NO

hν k1

F—ON

(F..NO)

k-1 hν k3 k-3

CT

k2

k4

F—ON

(R1)

F-NO

(R2)

The equilibrium between F-NO and F—ON could be expressed directly using a preequilibrium approximation for both reactions R1 and R2 (reaction R3):

F-NO

k1 k 2 k-1 k3 k 4 k-3

F—ON

(R3)

The equilibrium constant K for reaction R3 is thus (eq 4): ( ( ( (eq 4) 𝐾 = @ F GE (E (G@ (H

In eq. 4, k1

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