Chlorodifluoroacetyl Isothiocyanate, ClF2CC(O)NCS: Preparation and

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Chlorodifluoroacetyl Isothiocyanate, ClF2CC(O)NCS: Preparation, Structural and Spectroscopic Studies Luis Alejandro Ramos, Sonia Elizabeth Ulic, Rosana Mariel Romano, Yury Viktorovich Vishnevskiy, Norbert Werner Mitzel, Helmut Beckers, Helge Willner, Shengrui Tong, Maofa Ge, and Carlos Omar Della Védova J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp403549f • Publication Date (Web): 07 Jun 2013 Downloaded from http://pubs.acs.org on June 9, 2013

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The Journal of Physical Chemistry

Chlorodifluoroacetyl Isothiocyanate, ClF2CC(O)NCS: Preparation, Structural and Spectroscopic Studies

Luis A. Ramos,† Sonia E. Ulic,† Rosana M. Romano,† Yury V. Vishnevskiy,⊥ Norbert W. Mitzel,⊥ Helmut Beckers,# Helge Willner,# Shengrui Tong,§ Maofa Ge,§ and Carlos O. Della Védova†,*

†

CEQUINOR (UNLP-CONICET), Departamento de Química, Facultad de Ciencias Exactas,

Universidad Nacional de La Plata, 47 esq. 115, 1900 La Plata, República Argentina. #Fachbereich C - Anorganische Chemie, Bergische Universität Wuppertal, 42097 Wuppertal, Germany. §State Key http://www.youtube.com/watch?v=DJVtIcB4n8oLaboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ⊥

Universität Bielefeld, Lehrstuhl für Anorganische Chemie und Strukturchemie, Universitätsstraße 25, 33615 Bielefeld, Germany.

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ABSTRACT

Chlorodifluoroacetyl isothiocyanate, ClF2CC(O)NCS, was synthesized by the reaction of ClF2CC(O)Cl with excess of AgNCS. The colorless product melts at –85 ºC, and its vapor pressure follows the equation ln p = -4471.1 (1/T) + 11.35 (p [Atm], T [K]) in the range –38 to 22 ºC. The compound has been characterized by IR (gas phase, Ar matrix and matrix photochemistry), by liquid Raman, by

19

F and

13

C NMR, gas UV-Vis and photoelectron spectroscopy (PES), by

photoionization mass spectrometry (PIMS) and by gas electron diffraction (GED). The conformational properties of ClF2CC(O)NCS have been analyzed by joint application of vibrational spectroscopy, GED and quantum chemical calculations. The existence of two conformers has been detected in the gas and liquid phases, in which the C–Cl bond adopts a gauche orientation with respect to the C=O group; the C=O group is in syn- or anti-position with respect to the N=C double bond of the NCS group. The computed ∆G° difference between these two gauche-syn and gaucheanti forms is ∆G° = 0.63 kcal mol–1 in the B3LYP/6-31G(d) approximation. The most significant gas phase structural parameters for gauche-syn ClF2CC(O)NCS are re(NC=S) 1.559(2) Å, re(N=CS) 1.213(2) Å, re(N–C) 1.399(7) Å, re(C=O) 1.199(2) Å, ∠e(CNC) 134.7(13)°. Photolysis of ClF2CC(O)NCS using an ArF excimer laser (193 nm) mainly yields ClF2CNCS, CO and ClC(O)CF2NCS. The valence electronic properties of the title compound were studied using PES and PIMS. The experimental first vertical ionization energy of 10.43 eV corresponds to the ejection primarily of the sulfur lone-pair electrons of the in-plane nonbonding orbital on the NCS group.

KEYWORDS: Vibrational spectroscopy • matrix isolation • isothiocyanates • quantum chemical calculations • photoelectron and photoionization spectroscopy • gas electron diffraction


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INTRODUCTION

Several studies on thio- and isothiocyanates have been carried out recently related to their wide variety of medicinal, pharmaceutical and industrial applications along their valuable spectroscopic and electronic properties in ground and excited states. 1,2 Due to their stability isothiocyanates are important intermediate and final products in chemical industry.3 They are also present in a number of phytochemicals found in cruciferous vegetables such as broccoli, cauliflower, cabbage and Brussels sprouts. Consumption of cruciferous vegetables appears to be associated with a reduced risk of degenerative diseases such as cancer and cardiovascular illnesses.4 In particular, isothiocyanates inhibit the medicinal proliferation of tumor cells both in vitro and in vivo and are used as chemopreventive agents. The initiating event for these effects seems to be acute cellular stress caused by this class of compounds, although the elucidation of the mechanism is still at an early stage.5 Due to their volatility and stability, the isothiocyanates can reach the troposphere and react with radicals to give certain conversion products, thereby presenting a potential ecological risk.3 These outcomes must be related with their chemical nature and based on their electronic structures. Therefore, in this work we will combine several spectroscopic techniques and diffraction methods to gain a detailed understanding of the title species. Derivatives studied previously include fluorocarbonyl isothiocyanates,1 and other well-known carbonyl isothiocyanates.6,7,8,9 For ClF2CC(O)NCS, a hitherto unknown species, we will report the results from an interdisciplinary study including its preparation, determination of physical properties, its IR (gas phase, Ar matrix), liquid Raman, 19F and 13C NMR, gas UV-Vis, and photoelectron spectra (PES), photoionization mass spectra (PIMS) and its gas phase structure determined by electron diffraction (GED). These data will be complemented and compared with results from computations to analyze the conformational and photochemical behavior and the molecular energy levels of ClF2CC(O)NCS in its valence region.

EXPERIMENTAL SECTION Synthesis ClF2CC(O)NCS was synthesized by treatment of difluorochloroacetyl chloride, ClCF2C(O)Cl, with an excess of silver thiocyanate, AgSCN. For this purpose 1.0 g ClCF2C(O)Cl were distilled onto 1.4 g of dry AgSCN in a 250 ml glass vessel provided with a Young valve with PTFE stems (Young, London, U.K.). The reaction was carried out in vacuum for 1 hour at 15 ºC. Purification of the product was performed by trap-to-trap distillations with traps held at –47, –95 and –196 ºC. ClF2CC(O)NCS was mainly isolated in the trap at −47 ºC. The trap held at – 95 ºC also contained some of the product, which was further separated by slow distillation. The final yield was 95 %. ACS Paragon Plus Environment

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Silver thiocyanate was prepared from AgNO3 and KSCN, and ClCF2C(O)Cl by chlorination of the corresponding acid, ClCF2C(O)OH, (Merck & Co.), with PCl5.

Instrumentation and Procedure (a) General Procedure. Volatile materials were manipulated in a glass vacuum line equipped with a capacitance pressure gauge (221 AHS-1000, MKS Baratron, Burlington, MA), three U-traps and valves. The vacuum line was connected to an IR cell (optical path length 200 mm, Si windows 0.5 mm thick) placed in the sample compartment of a FTIR spectrometer (see below). This arrangement allows following the course of the reactions and the purification processes. The pure compound was stored in flame-sealed glass ampoules under liquid nitrogen in a Dewar vessel. The ampoules were opened with an ampoule key at the vacuum line, an appropriate amount was taken out for the experiments, and then they were flame-sealed again.10 The vapor pressures of the sample were measured in a small vacuum line equipped with a calibrated capacitance pressure gauge (MKS Baratron, AHS-100) and a small sample reservoir. The melting point of ClF2CC(O)NCS was determined by condensing 100 mg of the sample in vacuum near the bottom of a small L-shaped tube connected to a vacuum line. Then, the temperature was increased at a rate of about 1 ºC min–1 starting at –100 ºC (cold ethanol bath). The solid melts at –85 ºC to a colorless liquid. The vapor pressure of the liquid was measured with a precision thermometer, in a similar way, in the temperature range between –38 and 22 ºC. (b) Vibrational Spectroscopy. Infrared gas spectra were recorded on a Bruker Vector 25 spectrometer, with a resolution of 2 cm–1 in the range from 4000 to 400 cm–1. Raman spectra of neat liquids were measured at room temperature in flame-sealed capillaries (3 mm o.d.) on a FT Bruker RFS 106/S spectrometer, equipped with a 1064 nm Nd:YAG laser, in the region from 4000 to 100 cm–1 using a resolution of 2 cm–1. (c) Matrix isolation experiments. For matrix isolation experiments carried out in Wuppertal ClF2CC(O)NCS was diluted with argon in a ratio of 1:1000 in a 1 L stainless-steel storage container, and then small amounts of the mixture were deposited within 10 min onto the cold matrix support (16 K, Rh-plated Cu-block) in a high vacuum (10-5 Pa). Temperature-dependent conformational studies were carried out by passing the gaseous sample/Ar mixtures through a quartz nozzle (1 mm i.d.), heated over a length of ≈10 mm with a platinum wire (0.25 mm o.d.) prior to deposition on the matrix support. The nozzle was held at 298 or 583 K. IR spectra of matrix-isolated samples were recorded in a reflectance mode on a Bruker IFS 66v/S spectrometer using a transfer optic. A liquid N2 cooled HgCdTe detector (MCT) and a KBr/Ge beam splitter were used in the wavenumber range 5000 to 530 cm–1. For each spectrum with an


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apodized resolution of 0.25 cm–1 200 scans were added. More details of the matrix apparatus are given elsewhere.11 (d) UV Spectroscopy. The UV-visible spectrum of gaseous ClF2CC(O)NCS was recorded using a glass cell equipped with quartz windows (10 cm optical path length) on a Lambda EZ210 UV/Vis spectrometer (Perkin-Elmer). Measurements were carried out in the spectral region from 190 to 700 nm with a sampling interval of 1.0 nm, a scan speed of 200 nm min–1, and a slit of 2 nm. (e) NMR Spectroscopy. For

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C and 19F NMR spectra, pure samples were flame-sealed in thin-

walled 4 mm o.d. tubes and placed into 5 mm NMR tubes. The NMR spectra were recorded on a Bruker Avance 400 spectrometer at 100.6 and 376.5 MHz, respectively. The samples were held at 25 ºC and C6D6 was used as an external lock and reference. (f) Quantum Chemical Calculations. DFT calculations were performed using the program package GAUSSIAN 03.12 (MP2)13 calculations were carried out with the Firefly program.14 In the first step, the potential surface of internal rotations along the C2-C6 and C2-N3 bonds was calculated at the B3LYP/cc-pVTZ level of approximation by optimizing the molecular geometry with fixed torsion angles N3–C2–C6–Cl7 and O1–C2–N3–C4 in range from 0 to 360 and step size of 10 degrees (see Figure 1).

Figure 1. Potential surface for rotation along bonds C2–C6 and C2–N3 calculated on B3LYP/ccpVTZ level of theory. Only symmetry-unique minima were assigned.

In this way four low energy conformations were found: gauche-syn, gauche-anti, syn-syn and syn-anti (see Figure 2). Using the B3LYP/6-31G(d) and MP2(full)/aug-cc-pVTZ approximations full optimizations of geometries of these conformers were performed. Subsequent calculations of


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vibrational frequencies proved that they correspond to true minima on the potential energy surface. Thus, the reliability of the calculations is tested using different methods of approximation. For the selection of the method reported antecedents from the literature for similar compounds within selected applications are used. 1,5,9 (g) Photoelectron and Photoionization Mass Spectroscopy. The Beijing equipment used in this work has been described previously.15 The photoelectron and photoionization mass-spectrometer consists of two parts: the double-chamber UPS-II machine and a time-of-flight mass spectrometer. The PES was recorded in the double chamber UPS-II equipment with a resolution of about 30 meV as indicated by the Ar+ (2P3/2) photoelectron band. Experimental vertical ionization energies (PI) were calibrated by the simultaneous addition of a small amount of argon and methyl iodide to the sample. Mass analysis of ions was performed with the time-of-flight mass analyzer mounted directly to the photoionization region. The relatively soft ionization is provided by singlewavelength He I radiation. The PE and PIM spectra can be recorded successively within seconds under identical conditions. (h) Gas Electron Diffraction The electron diffraction patterns were recorded on the heavily improved Balzers Eldigraph KD-G2 gas-phase electron diffractometer16 at the University of Bielefeld. The experimental details are presented in Table 1. Two images for long and four images for short nozzle-to-plate camera distances were recorded on Fuji BAS-IP MP 2025 imaging plates. The plates with the diffraction patterns were scanned using a calibrated Fuji BAS-1800II scanner. The intensity curves (Figure S1, supporting information) were obtained by applying the method described earlier.17 Sector function and electron wavelengths were estimated using the method described in lit.18 and the benzene diffraction patterns, recorded along with that of the substance under investigation. Table 1. Details of the GED experiment

a

Parameter

Short camera

Long camera

Nozzle-to-plate distance, mm

250.0

500.0

Accelerating voltage, kV

60

60

Fast electrons current, µA

0.1

0.1

Electron wavelength,a Å

0.048733

0.048597

Nozzle temperature, K

293

293

Residual gas pressure,b mbar

2×10-5

2×10-5

Exposure time, sec

40

30

s range used, Å-1

6.2 – 34.0

2.2 – 15.0

Number of inflection pointsc

6

4

Determined from C6H6 diffraction patterns measured in the same experimental conditions.


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b

During the measurement.

c

Number of inflection points on the background line.

In order to obtain amplitudes of vibrations and curvilinear corrections used in the gas-phase electron diffraction refinements, analytical quadratic and numerical cubic force fields were calculated for all conformers employing the B3LYP/6-31G(d) approximation. The mean square amplitudes and vibrational corrections to obtain the equilibrium structure were computed with the SHRINK program.19,20,21,22

RESULTS AND DISCUSSION General Properties Solid ClF2CC(O)NCS melts at –85 ºC to a colorless liquid. The vapor pressure of the liquid was measured over a temperature range between –38 and 22 ºC. Its temperature dependence can be described by the equation (1) from which a boiling point of 121 ºC was extrapolated. ClF2CC(O)NCS is stable in the vapor and liquid phases at room temperature. ln p = –4471.1 (1/T) + 11.35 (p [Atm], T [K])

(1)

The ambient temperature 19F NMR spectrum shows a singlet at δ = –64.6 ppm attributed to both fluorine atoms of the ClCF2 group (Figure S2, supporting information). The 13C NMR spectrum exhibits two triplets at δ = 155.3 ppm (2J(C–F) = 36.9 Hz) and δ = 117.9 ppm (1J(C–F) = 302.9 Hz) and a singlet at 152.2 ppm, corresponding to the carbon atoms of the C=O, ClCF2 and NCS groups, respectively (Figure S3, supporting information). These values are in good agreement with those reported for similar compounds.1 The simplicity of both 19F NMR and 13C NMR spectra is the resultant of the rapid inter conversion between the rotamers being the signal positions originated by the weighted forms arising from the energetically most favorable gauche-syn and gauche-anti rotamers. The UV-visible spectrum of gaseous ClF2CC(O)NCS (Figure S4, supporting information) shows an absorption band at λmax = 274 nm attributed to the n→π* transition located on the NCS (?) chromophore and a stronger absorption at λmax = 199 nm, which could be due to the π→π* transition in the C=O chromophore, according to reported values for similar molecules.1 In Figure S4 spectra of CClF2C(O)NCO23 and CClF2C(O)N324 are included for comparison.

Computational chemistry The presence of both a chlorine atom and an NCS group in ClF2CC(O)NCS raises the possibility of more than one orientation around the C2–C6 and C2–N3 bonds (Figures 1 and 2). To evaluate the expected conformational equilibrium, the potential energy surface for the internal rotation ACS Paragon Plus Environment

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around both bonds was calculated at the B3LYP/cc-pVTZ level of theory (Figure 1). In addition, full optimizations of all possible conformers on the potential energy surface of ClF2CC(O)NCS were carried out using ab initio (MP2) and DFT (B3LYP) levels of theory. Four conformers, denoted as gauche-syn, gauche-anti, syn-syn and syn-anti, were found to have energy minima (Figure 1). The calculated energy differences of these rotamers and their estimated abundances at room temperature are given in Table 2. The energy differences were computed with respect to the lowest energy form for each theoretical method. For both methods the lowest energy rotamers present also the lowest free energy differences at 298 K. At the B3LYP/6-31G(d) level of theory the gauche-syn conformer was predicted to have less energy than the gauche-anti form, while the MP2(full)/aug-cc-pVTZ approximation gave the opposite results. All approximations result in dominance of gauche-syn and gauche-anti over syn-syn and syn-anti conformers.

Figure 2. Conformations of ClF2CC(O)NCS.

Table 2: Theoretical relative energy differences and abundances of the ClF2CC(O)NCS conformersa B3LYP/6-31G(d)

MP2(full)/aug-cc-pVTZ

g-s

g-a

s-s

s-a

g-s

g-a

s-s

s-a

∆E, kcal mol–1

0.00

0.67

1.41

1.47

0.53

0.00

1.80

1.24

∆Gº, kcal mol–1

0.00

0.63

1.08

1.26

0.27

0.00

1.36

1.04

xb, %

68

23

5

4

35

57

3

5

a

g-s, g-a, s-s and s-a denotations correspond to the gauche-syn, gauche-anti, syn-syn and syn-anti conformations,


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respectively. b Calculated from ∆Gº according to the Boltzmann law for the temperature 298.15 K.

Vibrational Spectra Gas-phase IR, argon matrix IR and Raman spectra of liquid ClF2CC(O)NCS are shown in Figure 3. The observed vibrational frequencies are compared in Table 3 with unscaled calculated harmonic frequencies for two possible conformers at the B3LYP/6-31G(d) level of theory. ClF2CC(O)NCS has 21 fundamental vibrational modes which are expected to be active in both IR and Raman spectra.

Figure 3. Upper trace: IR spectrum of ClF2CC(O)NCS isolated in an argon matrix (1:1000) at 15 K (resolution: 0.25 cm–1). Middle trace: IR spectrum of gaseous ClF2CC(O)NCS at 298 K (resolution: 2 cm–1). Lower trace: Raman spectrum of liquid ClF2CC(O)NCS at 298 K (resolution: 2 cm–1). A band due to HNCS is marked by an asterisk.


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Table 3. Experimental and calculated frequencies

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(cm–1) and assignments of the fundamental vibrational

modes of ClF2CC(O)NCS

Experimental Mode

ν1

ν2

ν3

IR

Raman

(Gas)[a]

(Ar Matrix)[b]

(liquid)[c]

1982 vs

1961.9

1951 vs

1939.1

1774 vs

1755.4

(6)

1747

(100)

1300.7

-

1260 w, sh

1263.7

1260

(6)

1176

(11)

1127

(20)

988

(28)

ν5

1130 s

ν6

995 m

ν9

1768.2

1955

1291 m

1188 s

ν8

1187.2 1181.0 1132.1 1126.1 992.8 984.0

Assignments[e]

gauche-syn

1814 (875)

ν(C=O) ν(C-C), ν(C-N)

1300 (182) 1255 (38)

νas(CF2)

1190 (292) 1183 (235)

νas(CF2)

1136 (153) 1130 (100) 977 (146)

νs(CF2)

874 (199)

νs(NCS), νs(CF2), ν(C-N)

964 (168)

873.4

870

(4)

839.5

839

(14)

851 (267)

730

(20)

740 (13) 689 (138)

710.4

νas(NCS)

1799 (399)

842 m

722.6

2058 (1460) 2025 (1806)

870 w, sh

721 vw

gauche-anti

688 w

680.2

675

(16)

562 vw

568.0

564

(56)

628.2

628

(36)

616.1

617

(45)

613 (40)

723 (40)

δ(CC(O)N) νs(C-N), δ(NCS), ν(CCl)

669 (27) 620 (31)

δ(CF2), ν(C-Cl)

508 (5)

540 (9)

δ(NCS)

498 (

Chlorodifluoroacetyl Isothiocyanate, ClF2CC(O)NCS: Preparation and

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