Crystal structure of methyl chloro­formate

The crystal structure of methyl chloro­formate, C₂H₃ClO₂, was determined by single-crystal X-ray diffraction data at 200 and 100 K. A suitable single crystal was grown in a capillary directly on the goniometer via Ostwald ripening around its melting point at 210 K. The ClC(O)OCH₃ mol­ecule has a staggered conformation and is planar (without H atoms) with point group C_S. In the crystal, methyl chloro­formate features weak Cl⋯O inter­actions forming chains along [100], which again extend into a network via inter­molecular C⋯O dipole inter­actions, consistent with Hirshfeld surface analysis. Complementary quantum chemical calculations at the DFT-def2-TZVP/PBE0-D3 level of theory were performed to compare with the experimental Raman data in both the liquid and solid state.

1. Chemical context

Methyl chloro­formate, ClC(O)OCH₃, is a liquid at room temperature with a melting point of 212 K (GESTIS, 2025). It is widely used in organic chemistry to introduce a meth­oxy­carbonyl functionality to a suitable nucleophile (Chiarucci et al., 2012). Methyl chloro­formate is well characterized, including its conformational properties by vibrational spectroscopy, microwave spectra (Groner et al., 1990), and gas phase electron diffraction (O'Gorman et al., 1950). Herein, we report on its hitherto unknown crystal structure on the basis of diffraction data recorded at 200 and 100 K. Additional quantum chemical calculations were performed and are consistent with the experimental findings and allow for the assignment of the vibrational modes.

2. Structural commentary

Methyl chloro­formate crystallizes in the ortho­rhom­bic crystal system with the centrosymmetric space group Pnma. It has been shown that crystals of small mol­ecular compounds can exhibit extensive polymorphism in the solid state, thus undergoing phase transitions at temperatures somewhere below their melting temperatures (Cruz-Cabeza et al., 2015). However, lowering the temperature starting from 200 K did not lead to a phase transition. The unit cell contains four formula units (Z = 4) and one independent mol­ecule in the asymmetric unit (Fig. 1). In the crystal structure, individual mol­ecules are arranged in an AB stacking pattern (Fig. 2).

Figure 1 Mol­ecular structure of a methyl chloro­formate mol­ecule in the solid state measured at 200 K (left) and 100 K (right). Atoms are drawn with displacement ellipsoids at the 70% probability level.

Figure 2 AB stacking in the crystal structure of methyl chloro­formate viewed along [010]. Repeating layers: A (left) and B (right).

In the solid state, ClC(O)OCH₃ adopts C_S symmetry with the mol­ecule lying on a mirror plane and thus displaying a torsion angle Cl1—C1—O2—C2 of exactly 180°. Due to the symmetry restrictions of point group C_S, the methyl group is in a staggered orientation with respect to the carbonyl group. Also, the O—CH₃ moiety is orientated syn relative to the C=O bond, with bond lengths similar to those determined by electron diffraction and comparable to those of phosgene (Zaslow et al., 1952) and dimethyl carbonate (Whitfield, 2023). A comparison of the structural parameters is given in Table 1.

Table 1
Comparison of selected inter­atomic distances (Å, °) of methyl chloro­formate (X-ray, this work), methyl chloro­formate (electron diffraction; O'Gorman et al., 1950), phosgene (Zaslow et al., 1952) and dimethyl carbonate (Whitfield, 2023)

	X-ray 200 K	X-ray 100 K	Electron diffraction	Phosgene	Dimethyl carbonate 82 K
C=O	1.186 (2)	1.195 (2)	1.19 (3)	1.15 (2)	1.219 (2)
C—Cl	1.7504 (13)	1.7502 (13)	1.75 (2)	1.74 (2)^*	–
(CO)—O	1.306 (2)	1.309 (2)	1.36 (4)	–	1.337 (2)^*
O—CH3	1.456 (2)	1.462 (2)	1.47 (4)	–	1.456 (2)^*
Cl—C=O	122.46 (10)	122.47 (10)	–	124(1.5)^*	–
Cl—C—O	108.61 (9)	108.75 (9)	112 (3)	–	–
O=C—O	128.93 (11)	128.78 (11)	126 (4)	–	125.58 (11)^*
^*Mean values.

3. Supra­molecular features

The crystal packing of methyl chloro­formate is dominated by short inter­molecular Cl⋯O contacts with a distance of d_Cl⋯O = 3.0442 (9) Å, forming infinite chains along [100] (Fig. 2). Within these chains, the mol­ecules are aligned in a zigzag fashion. However, individual chains are not inter­connected into layers, but instead form a framework through weak inter­molecular dipole inter­actions between the carbonyl oxygen atom and the central carbon atom of two parallel mol­ecules along [010].

A Hirshfeld surface analysis was performed using CrystalExplorer (Spackman et al., 2021) to qu­antify and visualize the inter­molecular inter­actions in the crystal structure of the title compound. The Hirshfeld surface was mapped with the d_norm function (Fig. 3), highlighting attractive inter­actions shorter than the sum of the van der Waals radii in red and equal or longer contacts in white and blue, respectively. Akin to the observations above, relevant contributions arise from Cl⋯O and C⋯O short contacts accounting for only 12.6% and 7.6% to the overall Hirshfeld plot. Other major contributions include Cl—H (35.0%) and O—H (27.4%) inter­molecular contacts. However, those are equal to or longer than the sum of their van der Waals radii.

Figure 3 Hirshfeld surface (top) and fingerprint plot (bottom) mapped for methyl chloro­formate on basis of 100 K data. Red areas indicate short contacts shorter than the sum of the van der Waals radii. Color code: C gray, O red, Cl green, H white.

4. Vibrational spectroscopy

Experimental Raman spectra of liquid and solid methyl chloro­formate were recorded at room temperature and at 153 K, confirming the mol­ecular structure as determined by X-ray analysis. The experimental spectra are confirmed by quantum chemical calculations at the DFT-def2-TZVP/PBE0-D3 level of theory (Weigend & Ahlrichs, 2005; Karttunen et al., 2015; Dovesi et al., 2018; Zicovich et al., 2004; Pascale et al., 2004; Maschio et al., 2013; Grimme et al., 2010) on the basis of the crystal structure of methyl chloro­formate. The recorded and calculated spectra are in good agreement, as shown in Fig. 4. The vibrational frequencies have been calculated within the harmonic approximation and therefore are overestimated, especially at higher wavenumbers. Nevertheless, the vibrational band at 2845 cm⁻¹ is not reproduced in the calculated spectrum. A comparison of calculated and observed vibrational bands is given in Table 2.

Table 2
Vibrational band positions (cm⁻¹) and band assignments of the Raman spectra of liquid and solid methyl chloro­formate based on the DFT calculation. Symmetry modes are given in parentheses.

νcalc.	vobs.(liquid)	vobs.(solid)	Assignment	νcalc.	vobs.(liquid)	vobs.(solid)	Assignment
182 (B2g)	–	195	out-of-plane δ(C—O—C)	1248 (Ag)	1206	1206	ν(C—O)+δ(CH3)
191 (B1g)				1253 (B3g)
272 (B3g)	281	283	in-plane δ(C—O—C)	1470 (B3g)	1457	1435	δs(CH3)
272 (Ag)				1474 (Ag)
409 (B3g)	407	411	in-plane δ(C—O—C—Cl)	1485 (Ag)		1452	δs(CH3)
410 (Ag)				1486 (B3g)
488 (Ag)	485	481	ν(C—Cl)	1491 (B1g)		1459	δas(CH3)
490 (B3g)		486		1494 (B2g)
719 (B1g)	–	–	out-of-plane νas(O—C—O) wagging	1875 (Ag)	1788	1756	ν(C=O)
723 (B2g)	–	–		1884 (B3g)		1778
845 (Ag)	823	824	δ(C—O—C—Cl) scissoring	3087 (Ag)	2967	2964	νs(CH3)
852 (B3g)				3088 (B3g)
996 (Ag)	950	950	ν(O—CH3)	3180 (B2g)	3026	3029	νas(CH3)
1004 (B3g)		975		3180 (B1g)
1174 (B1g)	1155	1148	δ(CH3) rocking	3222 (Ag)	3053	3056	νas(CH3)
1175 (B2g)				3222 (B3g)
1198 (Ag)		1155	δ(CH3) rocking
1212 (B3g)		1168
Notes: ν = stretching vibration, δ = bending vibration, s = symmetric, as = asymmetric, – = not observed.

Figure 4 Calculated (a) and recorded Raman spectra of methyl chloro­formate (b) at room temperature (liquid) and (c) at 153 K (crystalline). Calculated Raman spectra of solid ClC(O)OCH3 at the DFT-TZVP/PBE0 level of theory.

5. Synthesis and crystallization

Methyl chloro­formate (Fisher Scientific GmbH, 99%) was used as received. Its purity was checked with vibrational IR and Raman spectroscopy. A borosilicate glass capillary (0.3 mm, Hilgenberg) was filled with a small amount of methyl chloro­formate and flame-sealed at ambient pressure. The capillary was mounted onto the goniometer of the diffractometer and shock-cooled at 100 K using an open-flow cryostat to obtain a polycrystalline sample. The sample was incrementally heated at a rate of 180 K min⁻¹ until partial melting was observed at around 213 K. Subsequently, a suitable single crystal was grown through Ostwald ripening between 200 and 213 K in three cycles. Full datasets were collected at 200 and 100 K and reflections of the strongest scattering individuum integrated.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. The positions of the hydrogen atoms were obtained through difference-Fourier synthesis in both datasets.

Table 3
Experimental details

	100 K	200 K
Crystal data
Chemical formula	C2H3ClO2	C2H3ClO2
Mr	94.49	94.49
Crystal system, space group	Orthorhombic, Pnma	Orthorhombic, Pnma
a, b, c (Å)	8.4404 (5), 6.2019 (4), 7.4314 (4)	8.526 (2), 6.3454 (15), 7.4619 (16)
V (Å^3)	389.01 (4)	403.67 (16)
Z	4	4
Radiation type	Cu Kα	Cu Kα
μ (mm^−1)	7.23	6.97
Crystal size (mm)	0.30 × 0.05 (radius)	0.30 × 0.15 (radius)
Data collection
Diffractometer	Stoe Stadivari	Stoe Stadivari
Absorption correction	Multi-scan (LANA; Koziskova et al., 2016)	Multi-scan (LANA; Koziskova et al., 2016)
Tmin, Tmax	0.161, 0.337	0.231, 0.512
No. of measured, independent and observed [I > 2σ(I)] reflections	8421, 432, 415	10088, 453, 432
Rint	0.018	0.022
(sin θ/λ)max (Å^−1)	0.627	0.625
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.018, 0.049, 1.08	0.022, 0.064, 1.11
No. of reflections	432	453
No. of parameters	38	39
H-atom treatment	All H-atom parameters refined	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.18, −0.17	0.18, −0.16
Computer programs: X-AREA (Stoe, 2022), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

Supporting information

Computing details

Methyl chloroformate (srx03)

Crystal data

C2H3ClO2	Dx = 1.613 Mg m^−3
Mr = 94.49	Cu Kα radiation, λ = 1.54186 Å
Orthorhombic, Pnma	Cell parameters from 30155 reflections
a = 8.4404 (5) Å	θ = 5.3–75.7°
b = 6.2019 (4) Å	µ = 7.23 mm^−1
c = 7.4314 (4) Å	T = 100 K
V = 389.01 (4) Å^3	Block, clear colourless
Z = 4	0.30 × 0.15 × 0.15 × 0.05 (radius) mm
F(000) = 192

Data collection

Stoe Stadivari diffractometer	432 independent reflections
Radiation source: microfocus sealed X-ray tube, XENOCS GENIX 3D CU HIGH FLUX	415 reflections with I > 2σ(I)
Graded multilayer mirror monochromator	Rint = 0.018
Detector resolution: 5.81 pixels mm^-1	θmax = 75.4°, θmin = 8.0°
ω and φ scans	h = −8→10
Absorption correction: multi-scan (LANA; Koziskova et al., 2016)	k = −7→7
Tmin = 0.161, Tmax = 0.337	l = −9→9
8421 measured reflections

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.018	All H-atom parameters refined
wR(F^2) = 0.049	w = 1/[σ^2(Fo^2) + (0.0349P)^2 + 0.0604P] where P = (Fo^2 + 2Fc^2)/3
S = 1.08	(Δ/σ)max = 0.001
432 reflections	Δρmax = 0.18 e Å^−3
38 parameters	Δρmin = −0.17 e Å^−3
0 restraints

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cl1	0.72372 (4)	0.750000	0.33814 (4)	0.02357 (16)
O2	0.65970 (11)	0.750000	0.66663 (10)	0.0201 (2)
O1	0.44492 (10)	0.750000	0.48536 (12)	0.0235 (2)
C1	0.58441 (15)	0.750000	0.51257 (16)	0.0179 (3)
C2	0.55644 (18)	0.750000	0.82457 (17)	0.0238 (3)
H2A	0.4907 (18)	0.622 (2)	0.8235 (14)	0.033 (3)*
H2B	0.622 (2)	0.750000	0.919 (3)	0.037 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0234 (2)	0.0267 (2)	0.0206 (2)	0.000	0.00467 (10)	0.000
O2	0.0148 (5)	0.0269 (5)	0.0186 (5)	0.000	0.0001 (4)	0.000
O1	0.0174 (5)	0.0283 (5)	0.0246 (5)	0.000	−0.0034 (4)	0.000
C1	0.0190 (6)	0.0154 (6)	0.0193 (6)	0.000	0.0008 (5)	0.000
C2	0.0213 (7)	0.0327 (8)	0.0174 (6)	0.000	0.0014 (5)	0.000

Geometric parameters (Å, º)

Cl1—C1	1.7502 (13)	C2—H2A	0.968 (14)
O2—C1	1.3094 (15)	C2—H2A^i	0.968 (14)
O2—C2	1.4619 (15)	C2—H2B	0.90 (2)
O1—C1	1.1945 (15)
C1—O2—C2	114.37 (10)	O2—C2—H2A	109.6 (7)
O2—C1—Cl1	108.75 (9)	O2—C2—H2B	105.1 (12)
O1—C1—Cl1	122.47 (10)	H2A—C2—H2A^i	110.1 (17)
O1—C1—O2	128.78 (11)	H2A—C2—H2B	111.2 (9)
O2—C2—H2A^i	109.6 (7)	H2B—C2—H2A^i	111.2 (9)
C2—O2—C1—Cl1	180.000 (1)	C2—O2—C1—O1	0.000 (1)

Symmetry code: (i) x, −y+3/2, z.

Methyl chloroformate (srx02)

Crystal data

C2H3ClO2	Dx = 1.555 Mg m^−3
Mr = 94.49	Cu Kα radiation, λ = 1.54186 Å
Orthorhombic, Pnma	Cell parameters from 21811 reflections
a = 8.526 (2) Å	θ = 5.9–75.5°
b = 6.3454 (15) Å	µ = 6.97 mm^−1
c = 7.4619 (16) Å	T = 200 K
V = 403.67 (16) Å^3	Block, clear colourless
Z = 4	0.30 × 0.15 × 0.15 × 0.15 (radius) mm
F(000) = 192

Data collection

Stoe Stadivari diffractometer	453 independent reflections
Radiation source: microfocus sealed X-ray tube, XENOCS GENIX 3D CU HIGH FLUX	432 reflections with I > 2σ(I)
Graded multilayer mirror monochromator	Rint = 0.022
Detector resolution: 5.81 pixels mm^-1	θmax = 74.6°, θmin = 7.9°
ω and φ scans	h = −8→10
Absorption correction: multi-scan (LANA; Koziskova et al., 2016)	k = −7→7
Tmin = 0.231, Tmax = 0.512	l = −9→9
10088 measured reflections

Refinement

Refinement on F^2	Hydrogen site location: difference Fourier map
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.022	w = 1/[σ^2(Fo^2) + (0.0492P)^2 + 0.0148P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.064	(Δ/σ)max < 0.001
S = 1.11	Δρmax = 0.18 e Å^−3
453 reflections	Δρmin = −0.16 e Å^−3
39 parameters	Extinction correction: SHELXL-2018/3 (Sheldrick 2015b), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0081 (16)
Primary atom site location: dual

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cl1	0.27937 (4)	0.750000	0.33697 (4)	0.0485 (2)
O2	0.34065 (11)	0.750000	0.66399 (10)	0.0382 (3)
O1	0.55347 (11)	0.750000	0.48601 (12)	0.0472 (3)
C1	0.41624 (15)	0.750000	0.51182 (15)	0.0333 (3)
C2	0.44070 (19)	0.750000	0.82207 (18)	0.0465 (4)
H2A	0.377 (3)	0.750000	0.917 (3)	0.068 (6)*
H2B	0.505 (2)	0.627 (2)	0.8228 (15)	0.064 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0511 (3)	0.0547 (3)	0.0396 (3)	0.000	−0.01092 (11)	0.000
O2	0.0267 (5)	0.0527 (6)	0.0352 (5)	0.000	0.0021 (3)	0.000
O1	0.0336 (5)	0.0621 (7)	0.0458 (6)	0.000	0.0086 (4)	0.000
C1	0.0334 (6)	0.0313 (6)	0.0353 (7)	0.000	0.0015 (5)	0.000
C2	0.0412 (8)	0.0649 (9)	0.0333 (7)	0.000	−0.0021 (5)	0.000

Geometric parameters (Å, º)

Cl1—C1	1.7504 (13)	C2—H2A	0.89 (2)
O2—C1	1.3057 (15)	C2—H2B^i	0.953 (17)
O2—C2	1.4557 (16)	C2—H2B	0.953 (17)
O1—C1	1.1856 (16)
C1—O2—C2	114.55 (11)	O2—C2—H2B	109.9 (8)
O2—C1—Cl1	108.61 (9)	O2—C2—H2B^i	109.9 (8)
O1—C1—Cl1	122.46 (10)	H2A—C2—H2B	110.0 (10)
O1—C1—O2	128.93 (11)	H2A—C2—H2B^i	110.0 (10)
O2—C2—H2A	106.8 (14)	H2B—C2—H2B^i	110 (2)
C2—O2—C1—Cl1	180.000 (1)	C2—O2—C1—O1	0.000 (1)

Symmetry code: (i) x, −y+3/2, z.