Spectroscopic Properties, Conformation and Structure of Difluorothiophosphoryl Isocyanate in the Gaseous and Solid Phase.

Difluorothiophosphoryl isocyanate, F2P(S)NCO was characterized with UV/vis, NMR, IR (gas and Ar-matrix), and Raman (liquid) spectroscopy. Its molecular structure was also established by means of gas electron diffraction (GED) and single crystal X-ray diffraction (XRD) in the gas phase and solid state, respectively. The analysis of the spectroscopic data and molecular structures is complemented by extensive quantum-chemical calculations. Theoretically, the Cs symmetric syn-conformer is predicted to be the most stable conformation. Rotation about the P-N bond requires about 9 kJ mol-1 and the predicted existence of an anti-conformer is dependent on the quantum-chemical method used. This syn-orientation of the isocyanate group is the only one found in the gas phase and contained likewise in the crystal. The overall molecular structure is very similar in gas and solid, despite in the solid state the molecules arrange through intramolecular O⋅⋅⋅F contacts into layers, which are further interconnected by S⋅⋅⋅N, S⋅⋅⋅C and C⋅⋅⋅F contacts. Additionally, the photodecomposition of F2P(S)NCO to form CO, F2P(S)N, and F2PNCO is observed in the solid Ar-matrix.

Introduction

Covalent isocyanates, XNCO, are frequently used building blocks in synthetic and materials chemistry.1 Prominent industrially‐used examples are toluene diisocyanates and methylene diphenyl isocyanate, which are utilized for rigid and flexible polyurethane products as thermoplastic elastomers and thermoset resins.2 These isocyanates easily undergo self‐addition reactions in the form of dimers and trimers yielding uretdiones or isocyanurates, respectively. Isocyanates bonded to main‐group elements are of particular interest regarding their spectroscopic, conformational and structural properties. In recent years the synthesis and characterization of boryl isocyanates, R2BNCO,3 silyl isocyanates, R3SiNCO,4 alkyl isocyanates, (RNCO),5 sulphenyl isocyanates, RSNCO,6 acyl isocyanates, RC(O)NCO7 and sulphonyl isocyanates, RS(O)2NCO8 have been explored by experiment and theory. The latter two, namely the acyl and sulphonyl isocyanates, exhibit significantly different structural and spectroscopic properties than the respective alkyl or sulphenyl compounds with the atom bearing the isocyanate group in a lower oxidation state.

The intrinsic structural and conformational properties of phosphorus‐bonded isocyanates are of special interest, because phosphorylated urethanes and ureas are used as flame‐retarding materials.9 Furthermore, some α‐oxo‐isocyanates like phosphoryl isocyanates, R2P(O)NCO, show physiological activity and find applications in insecticides and drugs.10

Dichlorophosphanyl isocyanate (Cl2PNCO)11 and dichlorophosphoryl isocyanate [Cl2P(O)NCO]12 is the only complementary pair of phosphorus(III) and phosphorus(V) compounds bearing isocyanate groups that has been characterized so far, spectroscopically and structurally, both in the solid state as well as in the gas phase. Quantum‐chemical calculations predict syn‐ and anti‐conformations for both phosphorus compounds in the different oxidation states with barriers to interconversion lower than 3 kJ mol−1. In the syn‐conformer the NCO group is oriented to the same side of the P−N bond as the lone pair (lp) or the phosphoryl function for Cl2PNCO and Cl2P(O)NCO, respectively. Thus, in the syn‐conformers the dihedral angle (lp/O)PNC is 0° and in the respecttive anti‐conformers this angle is 180°. In the condensed phase, X‐ray diffraction revealed aggregation of single molecules by O⋅⋅⋅C contacts. For the phosphorus(III) compound aggregation into endless chains by interactions between the isocyanate oxygen atom and a carbon atom of the neighbouring molecule is observed. However, for the respective phosphorus(V) compound dimers are found in the solid state, made up by contacts of the phosphoryl oxygen atom to the isocyanate carbon atom of a neighbouring molecule.

In the gas phase, however, different behaviours were observed, namely dynamic ones. The isocyanate group of the phosphorus(III) compound Cl2PNCO exhibits a fully dynamic character, i. e. the NCO moiety adopts all orientations about the P−N bond relative to the lone pair at phosphorus, which were weighted and taken into account in the refinement of the GED diffraction pattern. In contrast, the gas‐phase structure of the of the phosphorus(V) compound Cl2P(O)NCO could be refined with a local dynamic model, i. e. small but significant deviations of the orientation of the isocyanate group for the syn‐ and anti‐conformation were taken into account.

The corresponding fluorinated pair F2PNCO13 and F2P(O)NCO14 have been investigated as well. Quantum‐chemical calculations proposed the existence of syn‐ and anti‐conformers for both compounds.15 Anyhow, the fluorinated phosphorus(III) compound F2PNCO was examined solely by gas‐phase electron diffraction revealing the exclusive presence of a syn‐conformer. And the phosphoryl compound was examined only by X‐ray diffraction of single crystals, which reveal a dimer formation comparable to the aggregation motive of Cl2P(O)NCO.

In this contribution we describe the structural properties and thus the influence of the sulphur atom of difluorothiophosphoryl isocyanate, F2P(S)NCO, in the solid state and in the gas phase. The investigation is complemented by the elucidation of the vibrational properties (IR and Raman) and its photodecomposition in an Ar‐matrix.

Results and Discussion

Quantum Chemical Calculations

In order to locate the possible conformers of F2P(S)NCO on the potential hypersurface, several energy profiles for the rotation of the isocyanate group around the P–N bond were calculated. The results of the energy curves are shown in Figure 1.

Figure 1

Potential energy scan for the rotation of the isocyanate group around the P–N bond based on different methods all using the cc‐pVTZ basis set. The upper marked box is provided stretched in the energy‐dimension in the box below.

The potentials were calculated at the different levels of theory and all predict the syn‐conformer to be the most stable. In the syn‐conformation, the isocyanate group is located coplanar to the thiophosphoryl moiety. However, the calculations using the MP226 ab initio method and the M06‐2X27 density functional theory predict a shallow minimum with a dihedral angle ϕ(SP−NC) of 180° (anti). Both possible conformers are depicted in Figure 2. The barrier for the interconversion calculated on the basis of the two mentioned methods is approximately 1.5 kJ mol−1 higher than the respective one for the B3LYP28 calculation, which is 7.5 kJ mol−1.

Figure 2

Optimized minimum structures of difluorothiophosphoryl isocyanate (1).

The potential energy scan of the oxide F2P(O)NCO14 features the same behaviour with a flat minimum about the anti‐conformation with the interconversion barrier being about 2 kJ mol−1 lower than in the case examined here. The analogous phosphorus(III) species F2PNCO shows only one stable conformer as well.13, 15 Dichlorophosphoryl isocyanate shows two stable conformers, syn and anti, and the barrier to rotation amounts to 2–3 kJ mol−1 depending on the method and basis set employed for its calculation.12 Furthermore, different basis set/method combinations predict different energetically preferred conformers for dichlorophosphanyl isocyanate.11 Thus, the high barrier for rotation and the strong preference of the syn‐conformer in F2P(S)NCO is due to the substitution of the phosphorus atom with both sulphur and fluorine.

Vibrational Spectra of F2P(S)NCO

The IR (gas and Ar‐matrix) and Raman (liquid) spectra of F2P(S)NCOek; are shown in Figure 3. The strongest IR bands in the IR spectra (gas: 2297 cm−1; Ar‐matrix: 2293 cm−1, Table 1) correspond to most characteristic asymmetric NCO stretching mode (νasym(NCO)). They are slightly lower in frequency than those in F2P(O)NCO (gas: 2309 cm−1; Ar‐matrix: 2307 cm−1)14 but close to those in Cl2P(O)NCO (gas: 2290 cm−1; Ne‐matrix: 2289 cm−1).12 In the Raman spectrum, it appears as a very weak band at 2286 cm−1. The symmetric NCO stretching mode (νsym(NCO)) occurs at 1439 and 1428 cm−1 in IR (Ar‐matrix) and Raman (liquid) spectra, respectively. The shift in the band positions indicates weak interactions involving the NCO moiety in the condensed phase. The two PF2 stretching modes, νsym(PF2) and νasym(PF2), located at 950 and 927 cm−1 in the IR spectrum of gaseous F2P(S)NCO as a broad band; they are quite close to the two well‐resolved bands in the Ar‐matrix at 947 and 919 cm−1. In the Raman spectrum, only two very weak bands at 940 and 911 cm−1 were observed. Interestingly, a weaker band at 793 cm−1 appears beside the IR band at 811 cm−1 for the P−N stretching mode, whereas, only one band at 814 cm−1 is present in the gas‐phase IR spectrum. This side band is more likely due to the matrix‐site effect rather than the presence of a second conformer, since the relative intensity of the two IR bands remains almost unchanged when the vacuum deposition of

Figure 3

Upper trace: IR spectrum of F2P(S)NCO isolated in an Ar‐matrix at 2.8 K (absorbance A, resolution: 0.5 cm−1). Middle trace: IR spectrum of gaseous F2P(S)NCO at 300 K (transmission T, resolution: 2 cm−1). Lower trace: Raman spectrum of liquid F2P(S)NCO at 300 K (Raman intensity I, resolution: 2 cm−1). Bands associated with CO2 are marked with asterisks.

Table 1

Experimentally observed and calculated vibrational frequencies (>400 cm−1) of F2P(S)NCO.

observed[a]			Calculated (I IR)	Assignment[c]
IR (gas)	IR (matrix)	Raman (liquid)	[I Raman][b]
2297 vs	2293.4 vs	2286 vw	2372 (1434) [5]	νasym(NCO)
1430 m	1439.2 m	1428 m	1486 (105) [22]	νsym(NCO)
950 s	946.5 s	940 vw	924 (338) [1]	νsym(PF2)
927 s	919.3 s	911 w	891 (142) [2]	νasym(PF2)
814 s	810.3 s	793 m	785 (312) [5]	ν(PN)
677 vw	663.5 m	659 s	656 (8) [7]	ν(PS)
603 w			619 (24) [<1]	δo.p.(NCO)
596 w		591 vs	589 (23) [22]	δi.p.(NCO)
425 w		419 s	411 (21) [2]	δ(PF2 )

[a] Band positions and intensities: vs very strong, s strong, m medium strong, w weak, vw very weak. [b] Calculated harmonic IR frequencies at the B3LYP/6‐311+G(3df) level of theory, IR intensities (km mol−1) in parentheses and Raman intensities (Å4 amu−1) in square brackets. [c] Tentative assignment based on the calculated vibrational displacement vectors of F2P(S)NCO.

Photodecomposition of F2P(S)NCO

Given the frequently observed photolytic CO‐elimination in covalent isocyanates R‐NCO (→RN+CO, R=OCNC(O)−,30 MeOC(O)−,31 Me2NC(O)−32), the photochemistry of F2P(S)NCO in solid Ar‐matrix was also studied. Upon an ArF excimer laser (193 nm) irradiation, decomposition of F2P(S)NCO occurs as evidenced by the depletion of its IR band (Figure 4, lower trace). As a result, the IR bands for CO (e, 2140.2 cm−1),33 F2P(S)N (b, 1184.2, 927.1 and 872.4 cm−1) and F2PNS (c, 1223.2, 841.2 and 823.9 cm−1)34 appear. Additionally, a new species displaying a distinguishable IR band at 2271.7 cm−1 (f) also forms, and it coincides with the strongest IR band for F2PNCO (2271 cm−1, Ar‐matrix).35 Therefore, sulphur‐elimination in F2P(S)NCO also happens under laser irradiation at 193 nm, whereas, no IR band for further CO‐elimination product (F2PN) could be identified among the photolysis products. The assignment for the IR band at 2201.3 cm−1 (labelled with an asterisk in Figure 3) remains unclear.

Figure 4

Lower trace: IR difference spectrum reflecting the change of Ar‐matrix isolated F2P(S)NCO upon an ArF laser photolysis (3 Hz, 3 mJ, 25 min). Upper trace: IR difference spectrum reflecting the change of the matrix upon subsequent 365 nm light irradiation (48 W, 30 min). The IR bands of F2P(S)NCO (a), F2P(S)N (b), F2PNS (c), F2PSN (d), CO (e), F2PNCO (f), CO2 (g) and an unknown species (*) are labelled.

Subsequent irradiation of the Ar‐matrix with UV‐light (365 nm) mainly results in the transformation of F2PNS (c) to F2PSN (d, 1177.0, 846.0 and 814.1 cm−1). Traces of F2P(S)NCO (a) reforms by recombining the singlet thiophosphoryl nitrene F2P(S)N with CO in same matrix cages. Similar photolytic CO‐association reactions have been previously observed for the phosphorus analogue of nitrous oxide (OPN+CO→OPNCO)36 and phenylborylene (PhB+CO→PhBCO).37

Gas‐Phase Structure

The structure of free molecules of F2P(S)NCO was determined by gas‐phase electron diffraction. The refinement of the gas‐phase structure was performed using a one‐conformer model based on the syn‐conformer as suggested by quantum‐chemical calculations. The refined model resulted in overall a disagreement factor R f of 2.6 %. The radial distribution curve is depicted in Figure 5 also containing some labelled interatomic distances of the syn‐conformer. Table 2 compares the structural parameters of the gas‐phase electron diffraction refinement of difluorothiophosphoryl isocyanate with the other phosphorus‐bonded isocyanates and azides. In the series of the investigated phosphorus‐bonded pseudohalides F2P(S)NCO is one of the two compounds whose gas‐phase structures were determined using a one‐conformer model. The other one is the related phosphorus(III) species F2PNCO.13 The isocyanate group in all cases is almost linear, i. e. ∡(NCO) is always larger than 170°, and the bond lengths in this unit do not vary much in the examples listed in Table 2.38 Comparing the P−S bond length to the analogous distances in PF2H(S), PClF2(S) and PBrF2(S) the reported r a values (1.876(3), 1.864(8) and 1.881(4) Å, respectively)39 the one determined for F2P(S)NCO (1.874(1) Å) is most similar to the length found for the hydrogen compound PF2H(S). The P−N bond length of F2P(S)NCO is also the shortest among the examples listed in Table 2. This confirms the effect of strongly electron‐withdrawing substituents40 and the heavy‐atom effect41 in shortening bond length due to lowering the anti‐bonding orbitals which is then more available for conjugation. The bond lengths of the isocyanate group are in good accordance to the previously determined parameters of the phosphorus bonded isocyanates.

Figure 5

Experimental (circles ○) and model (continuous line −) radial distribution functions of F2P(S)NCO (1). The difference curve is shown below. Vertical bars indicate interatomic distances of the syn‐conformer; selected ones are labelled.

Table 2

Structural parameters of the gas‐phase structure of F2P(S)NCO and related molecules.

	reference	r(P=X)	r(PN)	r(N=C)	r(C=O)	r(P−Y)	∡(XPN)	∡(PNC)[b]	∡(NCO)[b]	∡(Y−P−Y)
F2P(S)NCO, r g [c]	this work	1.874(1)	1.635(1)	1.223(1)	1.164(1)	1.539(1)	119.2(3)	136.0(4)	174.0(4)	98.3(2)
Cl2P(O)NCO, r g [c]	12	1.442(1)	1.646(1)	1.212(1)	1.157(1)	1.995(1)	118.3(2)	135.8(2)	175.3(2)	102.8(1)
Cl2PNCO, r g [c]	11		1.667(3)	1.203(1)	1.159(1)	2.045(1)		137.2(6)	171.5(6)	99.4(2)
F2P(O)N3, r h1 [d]	14	1.437(4)	1.657(2)	1.251(3)	1.130(2)	1.531(6)	118.9(3)	117.8(5)	172(2)	98.8(3)
F2PN3, r h1 [d]	15		1.716(3)	1.247(3)	1.127(3)	1.572(2)		117.7(5)	177(1)	96.4(2)
F2PNCO, r a [e]	13		1.683(6)	1.256(6)	1.168(5)	1.563(3)		130.6(8)		97.9(8)

[a] Distances r in Å, angles and dihedral angles in deg. [b] For the azides the analogous parameters are given. X is the chalcogen atom and Y is the halide bonded to phosphorus. [c] r g is the thermally averaged internuclear distance . [d] r h1 is the distance between average nuclear positions and derived from r a: r h1=r a+u 2/r a−r anh−Δr anh−δr, with r a see [e], r anh is a correction for anharmonic vibration, δr is a small correction due to centrifugal distortion and u being the vibrational amplitude. [e] r a is the internuclear distance directly accessible by GED <1/r>−1, r a=r g−u 2/r e (≅r g−u 2/r a) with u being the vibrational amplitude and r e being the equilibrium distance.

Solid‐State Structure

The technique of in situ crystallisation was applied to obtain a single crystal of F2P(S)NCO as the substance is a liquid at ambient conditions. Details of the procedure as well as crystallographic details are provided in the experimental section (Table 4). Figure 6 shows the molecular structure. Relative to the gas‐phase structure no significant variations of the geometrical parameters are observed for the solid‐state structure.

Table 4

Details of the gas‐phase electron diffraction experiment for F2P(S)NCO.

Parameters	short detector distance	long detector distance
noozle‐to‐plate distance, mm	250.0	500.0
accelerating voltage, kV	60	60
fast electron current, μA	1.52	1.52
electron wavelength,[a] Å	0.048770	0.048737
nozzle temperature, K	297	294
sample pressure,[b] mbar	6.1 ⋅ 10−6	3.2 ⋅ 10−6
residual gas pressure, [c] mbar	2.0 ⋅ 10−6	5.1 ⋅ 10−7
exposure time, s	15	10
used s range, Å–1	8.0–30.0	3.0–15.0
number of inflection points[d]	3	4
R f factor, %	3.11	1.94

[a] Determined from CCl4 diffraction patterns measured in the same experiment. [b] During the measurement. [c] Between measurements. [d] Number of inflection points on the background lines.

Figure 6

Molecular structure in the solid state of F2P(S)NCO. Right: view along the P−N bond confirming the C s symmetry. The symmetry operation for generating equivalent positions is (+x,−y,+z) for (').

In the crystal structure the isocyanate F2P(S)NCO molecule lies on a crystallographic mirror plane of symmetry with its thiophosphoryl moiety and the isocyanate group; it thus has C s symmetry. In the row of the previously investigated solid‐state structures of phosphorus pseudo halides, the thiophosphoryl isocyanate is the first example, which lies on a crystallographic mirror plane. This confirms the conformational stability of F2P(S)NCO predicted theoretically and observed experimentally in the gas phase. The structure of F2P(O)N3 in the solid state occupies C s symmetry within the error range.14 Concerning the geometrical parameters of the isocyanate group the N−C and C−O lengths are the same within error limits for the isocyanates bound to a phosphorus atom in the formal oxidation state +V. However, in the phosphorus(III) isocyanate Cl2PNCO11 the N−C bond is shorter and the C−O bond is longer in the solid state. The approximate linearity of the isocyanate group is also observed for the isocyanates listed in Table 3 with the angle ∡(NCO) being about 174° in all mentioned cases.

Table 3

Structural parameters [a] of F2P(S)NCO and related molecules in the solid state. X is the chalcogen atom and Y stands for the halide bonded to phosphorus.

	ref.	r(P=X)	r(P−N)	r(N=C)[b]	r(C=O)[b]	r(P–Y)	∡(XPN)	∡(PNC) ∡(PNN)	∡(NCO)/ ∡(NNN)	∡(Y−P−Y)	ϕ(OP−NC)/ ϕ(OP−NN)
F2P(S)NCO	this work	1.881(1)	1.627(1)	1.213(1)	1.149(1)	1.533(1)	120.6(1)	132.1(1)	174.8(1)	98.8(1)	0
Cl2P(O)NCO	12	1.455(1)	1.623(1)	1.213(2)	1.149(2)	1.987(1) / 1.985(1)	118.4(1)	135.2(1)	174.1(1)	103.6(1)	−38.1(1)
Cl2PNCO	11		1.688(2)	1.202(2)	1.160(2)	2.054(1) / 2.049(1)		136.1(2)	174.6(2)	99.9(1)
F2P(O)NCO	14	1.438(2)	1.615(2)	1.216(3)	1.146(3)	1.519(2) / 1.517(2)	119.9(1)	132.5(2)	174.2(3)	99.4(1)	−18.9(5)
F2P(O)N3[b]	15	1.442(2)	1.639(2)	1.225(3)	1.113(3)	1.525(2) / 1.527(2)	120.1(1)	117.3(2)	173.4(3)	98.6(1)	−0.2(3)
F2P(O)N3[b]	13	1.447(2)	1.638(2)	1.247(3)	1.117(3)	1.523(2) / 1.526(2)	120.1(1)	118.6(2)	172.4(3)	99.0(1)	−0.7(3)

[a] Distances r in Å, angles and dihedral angles in deg. [b] In this case two crystallographically independent molecules were found.

The most pronounced difference between the solid‐state structure of the sample investigated here and the other isocyanates are the observed O⋅⋅⋅F contacts (Figure 7). While in the other solid‐state structures interactions between either the phosphoryl or the isocyanate oxygen atom with the most electropositive carbon atom are observed, the thiophosphoryl isocyanate shows O⋅⋅⋅F contacts at 2.924(1) Å (sum of van der Waals‐radii: 2.96 Å).42 The short contact and the resulting normalized contact value43 of 0.98 give rise to the halogen bonding44 motif at the fluorine substituent with the electron donating oxygen atom. In general, halogen bonding at fluorine atoms is rarely observed except the fluorine substituent is bound to an extremely strong electron withdrawing group like the trifluoromethoxy (CF3O−) group,45, 46 yet it has been investigated as an amphiphilic partner in non‐covalent interactions.47 Despite the short interatomic distance, halogen bonding is characterized by the angle at the halogen atom to be nearly 180°. In the case examined here the P−F⋅⋅⋅O angle measures 143.8(1)° and is thus far from the presumed linearity for halogen bonding. On the other hand, the C=O⋅⋅⋅F angle of 127.5(1)° suggests, assuming an approximate sp2 hybridization of the oxygen atom, an interaction of the fluorine atom with the lone pair at the oxygen atom. This is affirmed by the sum of the angles at oxygen (2×∡(C=O⋅⋅⋅F)+∡(F⋅⋅⋅O⋅⋅⋅F)) being 350.0(3)° and thus the oxygen atom is in almost planar coordination. Thus this O⋅⋅⋅F interaction can be classified as a type I interaction.48

Figure 7

View on the unit cell along the crystallographic c‐axis showing the O⋅⋅⋅F contacts. The symmetry operations for generating equivalent positions are: (+x,−y,+z) for ('); ( +x, –y,+z) for (''); (+x,1–y,+z) for ('''); (− x, +y,+z) for ('''').

The layers built up by the mentioned O⋅⋅⋅F contacts arrange in an antiparallel manner with cancelling dipole moments. Connections to the next parallel layers are made up by S⋅⋅⋅N contacts (3.391(1) Å) shorter than the sum of the respective van der Waals radii (3.55 Å). Figure 8 depicts the arrangement of different layers.

Figure 8

View along the crystallographic b‐axis showing the assembly of parallel and anti‐parallel layers in the solid‐state structure of F2P(S)NCO. The symmetry operations for generating equivalent positions are: ( –x, –y,1–z) for ('); ( +x, –y,+z) for (''); (+x,−y,+z) for ('); (1–x,−y,1–z) for (''').

Furthermore, the anti‐parallel orientated layers are linked by S⋅⋅⋅C (3.510(1) Å) and C⋅⋅⋅F contacts (3.183(1) Å). Both distances are shorter than the sum of the van der Waals radii of the corresponding atoms, 3.66 Å for S⋅⋅⋅C and 3.23 Å for C⋅⋅⋅F.

Conclusion

The vibrational spectroscopy, photochemistry, conformation, and structure of difluorothiophosphoryl isocyanate, F2P(S)NCO, was extensively investigated by a series of techniques: UV/vis and NMR spectroscopy, IR spectroscopy in the gas and matrix, Raman spectroscopy of the liquid, gas electron diffraction and single crystal X‐ray diffraction. In line with the computationally predicted preference of a syn‐conformation in F2P(S)NCO, the gas‐phase electron diffraction finds solely the syn‐conformer. Its structure determined in the gaseous state is very similar to the structure of the molecules embedded into a single crystal as examined by X‐ray diffraction. In the solid state, the molecules arrange into layers through weak O⋅⋅⋅F contacts, and the layers are further linked by S⋅⋅⋅N, S⋅⋅⋅C and C⋅⋅⋅F contacts.

Experimental Section

Sample Preparation

Similar to the synthesis of F2P(O)NCO,14 difluorothiophosphoryl isocyanate, F2P(S)NCO was synthesized by the reaction of F2P(S)Cl with AgNCO. Freshly distilled F2P(S)Cl (0.14 g, 1 mmol) was condensed into a flask containing freshly dried AgNCO (0.51 g, 3 mmol) at −196 °C. The mixture was slowly warmed to room temperature and stirred for 10 h. The volatile part of the reaction mixture was then distilled by passing through three successive cold traps maintained at −70, −90 and −196 °C. Pure F2P(S)NCO was retained in the −90 °C trap as a colourless liquid. The quality of the sample was checked using gas phase IR and 31P NMR spectroscopy. Vapour pressures of the liquid in the temperature range from −80 to −50 °C were recorded, from which the vaporization enthalpy (ΔH vap) was determined to be 33.6±0.9 kcal mol−1.

IR, Raman, UV/vis, and NMR Spectroscopy

The gas‐phase IR spectrum was measured in an IR gas cell (optical path length 20 cm, Si windows, 0.5 mm thickness), which was fitted into the sample compartment of the FT‐IR instrument (Bruker, Tensor 27). Raman spectrum was recorded on a Bruker Equinox 55 FRA 106/S FT‐Raman spectrometer using a 1064 nm Nd:YAG laser (200 mW) with 200 scans at a resolution of 2 cm−1. The UV/vis spectrum of F2P(S)NCO recorded by Shimadzu UV 3600, which exhibits strong absorption at 197 nm. 31P NMR spectrum was measured in CDCl3 solution at room temperature using a Bruker Avance 400 spectrometer (242.8 MHz), a triplet at 34.5 ppm [1 J(31P19F)=1311 Hz] was observed for F2P(S)NCO.

Matrix IR Spectroscopy

Matrix IR spectra were recorded on a FT‐IR spectrometer (Bruker 70 V) in a reflectance mode using a transfer optic. A KBr beam splitter and liquid‐nitrogen‐cooled MCT detector were used in the mid‐IR region (4000–600 cm−1). For each spectrum, 200 scans at a resolution of 0.5 cm−1 were co‐added. The gaseous samples were mixed with argon gas (1 : 1000) in a 1 L stainless‐steel storage container. Then the mixture passed through an aluminium oxide furnace (o.d. 2.0 mm, i.d. 1.0 mm), and immediately deposited (2 mmol h−1) onto a Rh‐plated copper block matrix support (2.8 K) in a high vacuum (∼10−5 Pa). Photolysis was performed by using an ArF excimer laser (Gamlaser EX5/250, 3 Hz, 193 nm) and a high‐power flashlight (Boyu T648, 365 nm, 48 W).

A potential energy surface scan was performed on different levels of theory using the Dunning‐type basis set16 by rotating the isocyanate group around the P−N bond. Subsequent optimizations of the minimum structures were performed on the respective combination of method and basis set including the calculation of vibrational frequencies to assure true minima. All calculations were performed using the GAUSSIAN09 quantum‐chemical program at the version D.01.17

Gas‐Phase Electron Diffraction Experiment

The electron diffraction patterns were recorded on the heavily improved Balzers Eldigraph KD‐G2 gas‐phase electron diffractometer at Bielefeld University. Instrumental details are reported elsewhere.18 Experimental parameters are listed in Table 4. The electron diffraction patterns were measured on the Fuji BAS‐IP MP 2025 imaging plates, which were scanned by using calibrated Fuji BAS 1800II scanner. The intensity curves (Figure 9) were obtained by applying the method described earlier.19 Sector function and electron wavelength were refined using carbon tetrachloride diffraction patterns,20 recorded in the same experiment as the substance under investigation.

Figure 9

Experimental (○) and model (−) molecular scattering intensities and the respective differences (lower traces, experiment‐model) for long (upper curves) and short (lower curves) nozzle‐to‐detector distances for F2P(S)NCO (1).

Gas‐Phase Electron Diffraction Structural Analysis

The structural analysis was performed using the UNEX program.21 All refinements were performed using two averaged intensity curves simultaneously (Figure 9), one from the short and another from the long nozzle‐to‐detector distance. These were obtained by averaging intensity curves measured in independent experiments obtained at the same camera distance. The starting geometry for the refinement of the gas‐phase structure of difluorothiophosphoryl isocyanate in syn‐conformation was taken from the optimized structures of the M06‐2X/cc‐pVTZ calculations. The mean square amplitudes and vibrational corrections to the equilibrium structure were calculated with the Vibmodule program22 based on the M06‐2X/cc‐pVTZ calculations as well. Independent geometrical parameters and their groups in the least‐squares refinement are listed in the Supporting Information. The bonded distances were refined in three individual groups divided based on the peaks of the radial distribution function they are located under. The angles were refined separately except the pair of ∡(PNC) and ∡(NCO) which were grouped together. During the refinement, no restraints were used and the amplitudes from the quantum‐chemical calculations were not refined. Within a group, the differences were kept fixed at the computational level.

X‐Ray Crystallography

A single crystal of difluorothiophosphoryl isocyanate23 was grown in situ at 181 K and cooled to 171 K at a rate of 5 K h−1 and subsequently to 95 K with 200 K h−1. X‐Ray diffraction patterns were measured on a Rigaku Supernova diffractometer using MoKα (λ=0.71073 Å) radiation at 95.0(2) K. Using Olex2,24 the structure was solved and refined with the ShelX structure solution program and refinement package.25 Crystal and refinement details are provided in Table 5.

Table 5

Summary of crystallographic data for F2P(S)NCO.

chemical formula	CF2NOPS
M r	143.05
crystal system	monoclinic
space group	C2/m (No.12)
a (Å)	12.6600(1)
b (Å)	6.6550(1)
c (Å)	5.8831(1)
β (°)	91.9288(9)
V (Å3)	495.38(1)
Z / Z'	4 / 0.5
T (K)	95.0(2)
ρ calc (g cm−3)	1.918
μ (mm−1)	0.895
2Θ range [°]	6.44–120.15
Index range h	−30 to 30
Index range k	−16 to 15
Index range l	−14 to 114
Refl. collect.	130917
Indep. refl.	3941
R int	0.0358
Data/restraints/parameters	2871/0/145
R 1, I>2σ(I) / all data	0.0296/0.0381
wR 2, I>2σ(I) / all data	0.0918/0.0975
GoF	1.076
ρ max/min [e Å−3]	0.48/−0.69
CCDC	2006262

[a] R 1 is defined as Σ||F|–|F||/Σ|F| for I>2σ(I). [b] wR 2 is defined as [Σw{F 2–F 2}2/Σw(F 2)2]1 for I>2σ(I).

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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