Inertial Defect Research Articles

Conformational landscape and inertial defect of methoxyphenol isomers studied by mm-wave spectroscopy and quantum chemistry calculations.

Because methoxyphenols (MP) are emitted in significant quantities during biomass fires and contribute to the secondary organic aerosols formation which impacts the climate, their gas phase monitoring in the atmosphere is crucial and requires accurate rovibrational cross sections determined with a good knowledge of their ground state (GS) and vibrationally excited state (ES) molecular parameters. Therefore, the rotational spectra of the two isomers, 2-MP (guaïacol) and 4-MP (mequinol), have been measured in absorption and in emission at room temperature using a frequency multiplication chain and a mm-wave Fourier transform chirped-pulse spectrometer, respectively. Guided by quantum chemistry calculations, the conformational landscape has been characterised and the observation of only one rotamer in the spectra of 2-MP and 4-MP has been explained. For 2-MP, the most stable conformation is justified by an intramolecular O-H⋯OCH3 hydrogen-bond which has been characterised by a topology analysis of the electron density. In a global fit including more than 30 000 line assignments, rotational and quartic centrifugal constants of the GS and the three lowest energy ES have been determined allowing to reproduce the millimeter-wave spectra at the experimental accuracy. The same work has been performed on the cis-rotamer of 4-MP highlighting some perturbations marring the fit quality for two vibrationally ES. Finally, the isomeric dependence of the negative inertial defect ΔI agrees with that of the lowest energy out of plane mode ν45, and the variation of ΔI with the degree of vibrational excitation allows a fine estimation of v45 = 1 vibrational wavenumber.

Synthesis, microwave spectra, x-ray structure, and high-level theoretical calculations for formamidinium formate

An efficient synthesis of formamidinium formate is described. The experimental x-ray structure shows both internal and external H-bonding to surrounding molecules. However, in the gas phase, this compound occurs as a doubly hydrogen bonded dimer between formamidine and formic acid. This doubly hydrogen-bonded structure is quite different from the solid state structure. Microwave spectra were measured in the 6-14 GHz range using a pulsed-beam Fourier transform microwave (MW) spectrometer. The two nonequivalent N-atoms exhibit distinct quadrupole coupling. The rotational, centrifugal distortion, and quadrupole coupling constants determined from the spectra have the following values: A = 5880.05(2), B = 2148.7710(2), C = 1575.23473(13), 1.5 χaa (N1) = 1.715(3), 0.5(χbb-χcc) (N1) = -1.333(4), 1.5 χaa (N2) = 0.381(2), 0.25(χbb-χcc) (N2) = -0.0324(2), and DJ = 0.002145(5) MHz. The experimental inertial defect, Δ = -0.243 amu Å2, is consistent with a planar structure. Accurate and precise rotational constants (A, B, and C), obtained from the MW measurements, were closely reproduced, within 1%-2% of the measured values, with the M11 DFT theoretical calculations. Detailed comparison of the measured and calculated A, B, and C rotational constants confirms the planar doubly hydrogen bonded structure. The calculated nitrogen quadrupole coupling strengths of the monomer are quite different from either of the two nitrogen sites of the dimer. The poor agreement between measured and calculated quadrupole coupling strengths shows that the dimer is not locked in the equilibrium structure but is likely undergoing large amplitude vibrational motion of the hydrogen atoms moving between the N and O atoms involved in the hydrogen bonding.

Rotational spectra and nuclear quadrupole coupling constants of 4- and 2-iodoimidazole

Pure rotational spectra of 4-iodoimidazole and 2-iodoimidazole have been measured and assigned to determine rotational, centrifugal distortion and nuclear quadrupole coupling constants. Samples of each isomer were laser-vaporised from the surface of a solid target rod using the focussed pulse of a Nd:YAG laser prior to cooling via supersonic co-expansion with argon buffer gas. Microwave spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy between 2.0 and 18.5 GHz. Inertial defects of 4-iodoimidazole and 2-iodoimidazole were determined to be 0.0906(1) a.m.u. Å2 and 0.0814(2) a.m.u. Å2 respectively, consistent with rigid planar geometries for these molecules. The evaluated nuclear quadrupole coupling constants of iodine atoms imply that approximately 1% of an electron is transferred from the iodine lone pair into aromatic π orbitals when iodine is substituted for a hydrogen atom in either the 4- or 2-position. Results are compared with data for other halogenated aromatic molecules and discussed in context of the extent of conjugation across the range.

Full Conformational Landscape of 3-Methoxyphenol Revealed by Room Temperature mm-wave Rotational Spectroscopy Supported by Quantum Chemical Calculations.

Room temperature millimeter-wave rotational spectroscopy supported by high level of theory calculations have been employed to fully characterise the conformational landscape of 3-Methoxyphenol, a semi-volatile polar oxygenated aromatic compound precursor of secondary organic aerosols in the atmosphere arising from biomass combustion. While previous rotationally-resolved spectroscopic studies in the microwave and in the UV domains failed to observe the complete conformational landscape, the 70-330 GHz rotational spectrum measured in this study reveals the ground state rotational signatures of the four stable conformations theoretically predicted. Moreover, rotational transitions in the lowest energy vibrationally excited states were assigned for two conformers. While the inertial defect of methoxyphenol does not significantly change between conformers and isomers, the excitation of the methoxy out-of-plane bending is the main contribution to the non-planarity of the molecule.

High-resolution far-infrared synchrotron FTIR spectrum of the ν12 band of formamide-d1 (DCONH2)

The spectrum of the ν12 band of formamide-d1 (DCONH2) was recorded using a synchrotron Fourier transform infrared (FTIR) spectrometer coupled to the Australian Synchrotron THz/Far-IR beamline, with an unapodized resolution of 0.00096 cm−1 in the 350–210 cm−1 region. For the first time, rovibrational constants up to five quartic and two sextic terms were derived for the v12 = 1 state through the fitting of a total of 2072 far-infrared transitions using Watson's A-reduced Hamiltonian in the Ir representation with a root-mean-square (rms) deviation of 0.000073 cm−1. The band centre of the ν12 band of DCONH2 was found to be 289.3327553(47) cm−1 although the experimental uncertainty was limited to ±0.0002 cm−1. Ground state rovibrational constants of DCONH2 up to five quartic and two sextic constants were derived from a fit of 847 ground state combination differences (GSCDs) obtained from the infrared transitions of the ν12 band, together with 6 previously reported microwave transitions, with a rms deviation of 0.000108 cm−1. The ground state rotational constants (A, B, and C) of DCONH2 were improved while the ground state centrifugal distortion constants were accurately obtained for the first time. The uncertainty of the measured infrared lines was estimated to be ±0.0002 cm−1. From the ground state rotational constants, the inertial defect of DCONH2 was calculated to be 0.0169412(11) uÅ2.

Microwave Spectra, Structure, and the Aromatic Character of 1-Chloroborepin.

High resolution microwave spectra for the somewhat unstable compound 1-chloroborepin were measured in the 5-10 GHz range using a pulsed beam Fourier transform microwave spectrometer. Transitions were assigned and measured for three isotopologues, which include the most abundant isotopologue, 11B35Cl, and the less abundant 10B35Cl and 11B37Cl isotopologues. The molecular parameters (MHz) determined for the 11B35Cl isotopologue are A = 3490.905(35), B = 1159.38520(79), C = 870.59492(56), 1.5χaa (11B) = -0.220(22), 0.25(χbb - χcc) (11B) = -1.5300(99), 1.5χaa (35Cl) = -54.572(33), and 0.25(χbb - χcc) (35Cl) = 4.7740(79). The inertial defect is calculated to be Δ = -0.174 amu Å2 from the experimental rotational constants, indicating a planar structure with some out of plane vibrational motion. An extended Townes-Dailey analysis was performed on the 11B and 35Cl nuclei to determine the electron occupations in the valence hybridized orbitals using the experimental quadrupole coupling strengths. From the analysis it was determined that Cl is sharing some electron density with the empty p-orbital on B. The B-Cl bond length determined from the data is 1.798(1) Å, and the B-C bond lengths are 1.533(10) Å. The structural parameters and electronic structure properties of 1-chloroborepin are consistent with an aromatic boron-containing molecule.

Infrared observation of OC-C2H2, OC-(C2H2)2 and their isotopologues

ABSTRACTThe fundamental band for the OC-C2H2 dimer and two combination bands involving the intermolecular bending modes ν9 and ν8 in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tuneable infrared quantum cascade laser. Analogous bands for OC-C2D2 and the fundamental for OC–DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C2H2)2. The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C2H2. A similar band for the fully deuterated isotopologue CO-(C2D2)2 confirms our assignment.

Oxygen-18 Isotopic Studies of HOOO and DOOO.

Owing to questions that still persist regarding the length of the O-H and central O-O bond, and large-amplitude torsional motion of trans hydridotrioxygen HOOO, a weakly bound complex between OH and O2, new 18O isotopic measurements of HOOO and DOOO were undertaken using Fourier transform microwave and microwave-millimeter-wave double resonance techniques. Rotational lines from three new 18O species of DOOO (D18OOO, DO18O18O, and D18O18O18O) were detected, along with the two singly substituted 18O isotopic species of HOOO (HO18OO and HOO18O) that were not measured in the previous isotopic investigation. From a least-squares fit, spectroscopic constants, including the three rotational constants, were precisely determined for all five species. The inertial defect of DOOO and its 18O species is uniformly negative: of order -0.04 amu Å2, regardless of the number or location of the 18O atoms, in contrast to that found for HOOO or its 18O isotopic species. A reanalysis of the molecular structure was performed using either normal HOOO and its four singly substituted isotopic species, the new DOOO data, or all the isotopic species (10 in total). The differences between the purely experimental (r0) structures are generally quite small, of order ±0.01 Å for the bond lengths and ±1° for the bond angle. The length of the O-H bond remains unrealistically short compared to free OH, and the central O-O bond length is consistently very close to 1.68 Å. On the basis of the effective O-H bond length derived from the experimental structure, the average displacement of the large amplitude torsional motion from planarity is estimated to be ∼22°.

Rotational spectrum and structure of the T-shaped cyanoacetylene carbon dioxide complex, HCCCN⋯CO2

The rotational spectrum of the T-shaped cyanoacetylene carbon dioxide dimer, HCCCN⋯CO2, was measured using two Balle-Flygare Fourier transform microwave (FTMW) spectrometers between 1.4GHz and 25GHz. Only the Ka=0, 2, 4 branches of spectrum from J′=1←0 to J″=16←15 transitions were observed. The vanishing of the Ka=1, 3, … transitions demonstrates a C2v symmetry complex with a T-shaped alignment of the subunits. The spectroscopic constants were fit using Pickett’s SPFIT/SPCAT suite of programs obtaining: A0=11273(18)MHz, B0=764.088(21)MHz, C0=716.254(21)MHz, ΔJ=0.50329(34)kHz, ΔJK=0.120867(11)MHz, ΔK=−28.17(36)MHz, δJ=0.0613(21)kHz, δK=44.25(95)kHz, ΦJ=0.0053(12)Hz, ΦJK=9.820(55)Hz, ΦKJ=−0.59325(72)kHz, ΦK=−2.3719(53)MHz, ϕJ=0.0398(42)Hz, ϕJK=6.9(9)Hz, and ϕK=−3.592(13)kHz. The 14N nuclear quadrupole coupling constants were fit to χaa=−4.12753(38)MHz and χbb−χcc=0.103(15)MHz. The small negative inertial defect, Δ0=−0.66(12)uÅ2, indicates a vibrationally averaged planar complex with non-negligible low frequency out-of-plane vibrations. While maintaining near-planar orientation, both binding partners exhibit large-amplitude bending vibrations within the plane. To deal with the intermolecular dynamics, a torsional oscillation model was developed in this work for the structural analysis. According to this model, the vibrational bending amplitude for HCCCN torsional angle is 10.(1)°, with the a-axis of complex; CO2 subtending a 5.4(5)° torsional oscillation angle with the b molecular axis. The van der Waals bond length is 3.0137(3)Å. The stretching force constant, ks=3.9N/m, and the stretching frequency, νs=53cm−1, for the van der Waals bond were calculated using the pseudo-diatomic model. High-level MP2 and DFT calculations of structural parameters, rotational constants, and 14N quadrupole coupling strengths were made and the results compared with experimental results.

Structure and dynamics of cyclic amides: The rotational spectrum of 1,3-dimethyl-2-imidazolidinone

The structure and the internal dynamics of the lactam 1,3-dimethyl-2-imidazolidinone, also known as N,N′-dimethylethyleneurea, have been investigated through the analysis of its free-jet absorption rotational spectrum. One conformer has been assigned. The pure μb-type spectrum, recorded in the 59.6–74.4GHz frequency range entails an inertial defect Δc=−16.39uÅ2, indicating that the molecule has C2 symmetry with a twisted arrangement of the ring. The methyl internal rotation barrier V3=7.181 (3)kJmol−1 and the 14N diagonal nuclear quadrupole coupling constants χaa=2.14 (14) and (χbb-χcc)=7.26 (6)MHz were determined from the analysis of the hyperfine structure. They are in good agreement with the ab initio MP2/6-311++G(d,p) calculations which also estimate the electric dipole moment value as 3.9 D.

Microwave spectrum, molecular structure, dipole moment, and quantum chemical calculations of s-trans-(E)-2-methyl-2-propenal oxime

The spectroscopic constants of s-trans-(E)-2-methyl-2-propenal oxime (methacryl- aldehyde oxime) of normal, H2CC(CH3)CHNOH, and its deuterated species, H2CC(CH3)CHNOD, were determined by observing their microwave spectra in the frequency range of 8 to 40GHz in the ground vibrational state. The rotational constants were A=8321.38(82), B=2076.09(1), and C=1678.60(1) MHz for normal species and A=8283.7(16), B=1998.63(2), and C=1626.21(2) MHz for deuterated species, respectively. The inertial defects (ΔI=Ic−Ia−Ib) of normal and deuterated species were determined to be −3.09(2) and −3.10(3)uÅ2, respectively. The dipole moments were determined as μa=0.53(2), μb=0.27(2), and μtotal=0.60(5) D. The 14N nuclear quadrupole coupling constants were determined as χaa=3.62(12), χbb=−5.1(14), and χcc=1.48(26)MHz. The comparison of the observed spectroscopic parameters with the calculated ones led to the conclusion that the assigned spectrum was due to s-trans-(E) form. The rs coordinates of the hydrogen atom in a hydroxyl group were determined and the OH bond was found to be at the trans position with respect to the CN double bond. The structural parameters of r(C2C3), r(C2C6), ∠C2C3N and ∠C6C2C3, for s-trans-syn form were adjusted to the four observed rotational constants (B and C). The observed rotational constants for s-trans-(E) form were in good agreement with those calculated using the MP2/6-31G (d,p) level of theory.

The radio spectra of planar aromatic heterocycles: how to quantify and predict the negative inertial defects.

The simplest tricyclic aromatic nitrogen heterocyclic molecules 5,6-benzoquinoline and 7,8-benzoquinoline are possible candidates for detection of aromatic systems in the interstellar medium. Therefore the pure rotational spectra have been recorded using frequency-scanned Stark modulated, jet-cooled millimetre wave absorption spectroscopy (48-87 GHz) and Fourier Transform Microwave (FT-MW) spectroscopy (2-26 GHz) of a supersonic rotationally cold molecular jet. Guided by theoretical molecular orbital predictions, spectral analysis of mm-wave spectra, and higher resolution FT-MW spectroscopy provided accurate rotational and centrifugal distortion constants together with 14N nuclear quadrupole coupling constants for both species. The tricyclic frames of these species undergo low energy out-of-plane zero-point vibrations resulting in deviations from the moments of inertia that the rigid structure would exhibit. The determined inertial defects, along with those of similar species are used to develop an empirical formula for calculation of inertial defects of aromatic ring systems. The predictive ability of the formula is shown to be excellent in general for planar species with a number of pronounced out-of-plane vibrations. The resultant constants for the benzoquinolines are of sufficient accuracy to be used in astrophysical searches for planar aromatic heterocycles.

Newly assigned microwave transitions and a global analysis of the combined microwave/millimeter wave rotational spectra of 9-fluorenone and benzophenone

Microwave spectra of 9-fluorenone and benzophenone have been observed using a broadband chirped-pulse Fourier Transform Microwave (cp-FTMW) Spectrometer. An analysis of the microwave spectra allowed for the assignment of 178 b-type rotational transitions for 9-fluorenone in the 8.0–13.0GHz region, the assignment of 166 b-type transitions for benzophenone in the 8.0–14.0GHz region, and effectively quadrupled the total number of pure rotational transitions observed for these molecules. This new microwave data and the previously published millimeter wave data of Maris et al. have been analyzed together in a global fit, where the resulting rotational constants accurately reproduce the spectra over the entire 8–80GHz region for both molecules. In addition, the resulting constants have been found to be consistent with the expected planar C2v structure for 9-fluorenone and the paddle-wheel like C2 structure of benzophenone. The rotational constants of the combined global fit have allowed for a more precise determination of the inertial defects (Δ) and second moments of inertia (Pcc) for 9-fluoreneone and benzophenone. Specific focus has been paid to the second moment of benzophenone, which when used in conjunction with theory strongly suggests an ∼32.9° torsional angle out of the ab-plane for the paddle-wheel structure of the gas-phase molecule.

Rovibrational constants of the ground state and v8 = 1 state of 13C2HD3 by high-resolution FTIR spectroscopy

The Fourier transform infrared (FTIR) spectrum of the c-type ν8 band of 13C2HD3 was recorded for the first time at a unapodized resolution of 0.0063cm−1 in the wavenumber region of 830–1000cm−1. Through the fitting of a total of 1057 assigned infrared transitions using Watson’s A-reduced Hamiltonian in the Ir representation, rovibrational constants for the upper state (v8=1) up to five quartic centrifugal distortion terms were derived for the first time with a root-mean-square (rms) deviation of 0.00073cm−1. The band center of ν8 of 13C2HD3 was found to be 913.011021(55)cm−1. Ground state rovibrational constants up to five quartic terms of 13C2HD3 were also determined from a fit of 453 ground state combination-differences from the present infrared measurements with an rms deviation of 0.00072cm−1 for the first time. The uncertainty of the measured infrared lines was estimated to be ±0.0012cm−1. From the ground state rotational constants, the inertial defect of 13C2HD3 was calculated to be 0.06973(16)uÅ2, showing the high planarity of the molecule.

The pure rotational spectrum of a Claisen rearrangement precursor Allyl Phenyl Ether using CP-FTMW spectroscopy

The pure rotational spectrum of a Claisen rearrangement precursor, Allyl Phenyl Ether (APE), has been measured on a chirped pulse Fourier transform microwave (CP-FTMW) spectrometer in the 8–14GHz region. Rotational and centrifugal distortion constants for multiple conformations have been determined and are reported for the first time. This is the first study of a phenyl-containing ether where multiple conformers were experimentally observed all within their ground vibrational states. Quantum chemical calculations have been performed to isolate low energy geometries of APE and are implemented to aid in spectral assignment. Other structural parameters such as planar moments and inertial defects for the Allyl Phenyl Ether conformers are presented and compared to similar molecules.

Microwave spectra, molecular structure, and aromatic character of 4a,8a-azaboranaphthalene.

The microwave spectra for seven unique isotopologues of 4a,8a-azaboranaphthalene [hereafter referred to as BN-naphthalene] were measured using a pulsed-beam Fourier transform microwave spectrometer. Spectra were obtained for the normal isotopologues with (10)B, (11)B, and all unique single (13)C and the (15)N isotopologue (with (11)B), in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the (11)B(14)N isotopologue are A = 3042.712 75(43) MHz, B = 1202.706 57(35) MHz, C = 862.220 13(35) MHz, DJ = 0.06(1) kHz, 1.5χaa ((14)N) = 2.5781(61) MHz, 0.25(χbb - χcc) ((14)N) = - 0.1185(17) MHz, 1.5χaa (11B) = - 3.9221(75) MHz, and 0.25(χbb - χcc) ((11)B) = - 0.9069(24) MHz. The experimental inertial defect is Δ = - 0.159 amu Å(2), which is consistent with a planar structure for the molecule. The B-N bond length from the experimentally determined structure is 1.47 Å, which indicates π-bonding character between the B and N. The measured quadrupole coupling strengths provide important and useful information about the bonding, orbital occupancy, and aromatic character for this aromatic molecule. Extended Townes-Dailey analyses were used to determine the B and N electron sp(2)-hybridized and p-orbital occupations. These results are compared with electron orbital occupations from the natural bond orbital option in theoretical calculations. From the analyses, it was determined that BN-naphthalene has aromatic character similar to that of other N-containing aromatics. The results are compared with similar results for B-N bonding in 1,2-dihydro-1,2-azaborine and BN-cyclohexene. Accurate and precise structural parameters were obtained from the microwave measurements on seven isotopologues and from high-level G09 calculations.

Rotational spectra and gas phase structure of the maleimide – Formic acid doubly hydrogen bonded dimer

Rotational transitions were measured for the maleimide – formic acid doubly hydrogen bonded dimer using a Flygare–Balle type pulsed-beam Fourier transform microwave spectrometer. No splittings caused by possible concerted double proton tunneling motion were observed. Experimental rotational constants (MHz), quadrupole coupling constants (MHz), and centrifugal distortion constants (kHz) were determined for the parent and three deuterium substituted isotopologues. The values for the parent are A=2415.0297(10), B=784.37494(38), C=592.44190(33), DJ=0.0616(64), DJK=−0.118(35), DK=−1.38(15), 1.5χaa=2.083(14), and 0.25(χbb–χcc)=1.1565(29). The hydrogen bond lengths were determined using a nonlinear least squares structure fitting program. Rotational constants for this complex are consistent with a planar structure, with an inertial defect of Δ=−0.528amuÅ2. The B3LYP calculation yielded rotational constants within 0.1% of the experimental values.

Microwave spectrum and gas phase structure of maleimide

The rotational spectrum of maleimide was measured in the 5–12GHz range using a Flygare–Balle type, pulsed-beam Fourier transform microwave spectrometer. Rotational transitions were measured for the parent, all unique singly substituted 13C isotopologues, and an ND, deuterium substituted isotopologue. The parent (or normal isotopologue) rotational constants, centrifugal distortion constants, and quadrupole coupling constants are A=6815.3251(12)MHz, B=2361.85011(64)MHz, C=1754.32750(64)MHz, DJ=0.232(24)kHz, DJK=0.546(54)kHz, 1.5χaa=2.4227(53)MHz, and 0.25(χbb–χcc)=1.3679(15)MHz. A best fit gas phase structure was determined using the experimental rotational constants of the isotopologues and some parameters from calculations. The inertial defect is Δ=−0.054amuÅ2, indicating a planar structure for maleimide, with no large amplitude motions observed on the NH hydrogen atom. Calculations using B3LYP/aug-cc-pVTZ provided rotational constants which are much closer (within 1–2%) to the experimental values compared to the MP2/aug-cc-pVTZ calculated values.

Microwave spectrum and the gas phase structure of phthalimide

The microwave spectrum of phthalimide (PhI) was measured in the 4.8–9.5GHz range using a Flygare-Balle type, pulsed-beam Fourier transform microwave spectrometer. Rotational transitions were measured for the parent and all unique single 13C substituted isotopologues. The rotational (MHz), centrifugal distortion (kHz), and quadrupole coupling constants (MHz) were determined for the parent to be A=1745.6655(10), B=1199.3309(6), C=711.0864(3), DJ=0.012(7), DJK=−0.05(9), 1.5χaa=2.719(11), and 0.25(χbb−χcc)=1.236(3). Using the measured rotational constants of the isotopologues, a nonlinear least squares fit was performed to obtain the best fit gas phase structure. The inertial defect is Δ=−0.175amuÅ2, indicating a planar structure for PhI.Calculations using B3LYP/aug-cc-pVQZ provided rotational constants which are much closer to the experimental values compared to the MP2/6-311++G∗∗ calculated values.

High-resolution FTIR spectroscopy of ethylene-d3 (C2HD3): Rovibrational constants for the ground state and the v8 = 1 state

The Fourier transform infrared (FTIR) spectrum of the v8 band of ethylene-d3 (C2HD3) was recorded with a unapodized resolution of 0.0063cm−1 in the wavenumber region of 830–1000cm−1. Improved rovibrational constants for the upper state (v8=1) up to five quartic centrifugal distortion terms were derived by assigning and fitting a total of 1249 infrared transitions using Watson’s A-reduced Hamiltonian in the Ir representation. The root-mean-square (rms) deviation of the fit was 0.00062cm−1, close to the accuracy (1/10th of a linewidth) of the measurements of the line positions of ±0.0007cm−1. The ground state rovibrational constants of C2HD3 were also determined with higher accuracy than previously reported from a fit of 568 combination-differences from the present infrared measurements with an rms deviation of 0.00038cm−1, using Watson’s A-reduced Hamiltonian. The calculated inertial defect is 0.069489(69)μÅ2. The C-type v8 band of C2HD3 was determined to be centered at 918.731925(40) cm−1. The ground state constants of C2HD3 derived from the experimental GSCD fits were in reasonable agreement with the equilibrium state constants derived from theoretical calculations using the B3LYP/cc-pVTZ, MP2/cc-pVTZ, and CCSD(T)/cc-pVTZ levels of theory.