A-type Transitions Research Articles

CN and CCH derivatives of ethylene and ethane: Confirmation of the detection of CH3CH2CCH in TMC-1

We present a study of CH3CH2CCH, CH3CH2CN, CH2CHCCH, and CH2CHCN in TMC-1 using the QUIJOTE line survey. We confirm the presence of CH3CH2CCH in TMC-1, which was previously reported as tentative by our group. From a detailed study of the ethynyl and cyanide derivatives of CH2CH2 and CH3CH3 in TMC-1, we found that the CH2CHCCH/CH2CHCN and CH3CH2CCH/CH3CH2CN abundance ratios are 1.5±0.1 and 4.8±0.5, respectively. The derived CH2CHCCH/CH3CH2CCH abundance ratio is 15.3±0.8, and that of CH2CHCN over CH3CH2CN is 48±5. All the single substituted isotopologs of vinyl cyanide have been detected, and we found that the first and second carbon substitutions in CH2CHCN provide a 12C/13C ratio in line with that found for other three-carbon bearing species such as HCCNC and HNCCC. However, the third 13C isotopolog, CH2CH13CN, presents an increase in its abundance similar to that found for HCCCN. Finally, we observed eight b-type transitions of CH2CHCN, and we find that their intensity cannot be fitted adopting the dipole moment µb derived previously. These transitions involve the same rotational levels as those of the a-type transitions. From their intensity, we obtain µb = 0.80±0.03 D, which is found to be in between earlier values derived in the laboratory using intensity measurements or the Stark effect. Our chemical model indicates that the abundances of CH3CH2 CCH, CH3CH2CN, CH2CHCCH, and CH2CHCN observed in TMC-1 can be explained in terms of gas-phase reactions.

First Detection in Space of the High-energy Isomer of Cyanomethanimine: H2CNCN

We report the first detection in the interstellar medium of N-cyanomethanimine (H2CNCN), the stable dimer of HCN of highest energy and the most complex organic molecule identified in space containing the prebiotically relevant NCN backbone. We have identified a plethora of a-type rotational transitions with 3 ≤ J up ≤ 11 and K a ≤ 2 that belong to this species toward the Galactic center G+0.693-0.027 molecular cloud, the only interstellar source showing the three cyanomethanimine isomers (including the Z- and E-isomers of C-cyanomethanimine, HNCHCN). We have derived a total column density for H2CNCN of (2.9 ± 0.1) × 1012 cm−2, which translates into a total molecular abundance with respect to H2 of (2.1 ± 0.3) × 10−11. We have also revisited the previous detection of E- and Z-HNCHCN and found a total C/N-cyanomethanimine abundance ratio of 31.8 ± 1.8 and a Z/E-HNCHCN ratio of 4.5 ± 0.2. While the latter can be explained on the basis of thermodynamic equilibrium, chemical kinetics are more likely responsible for the observed C/N-cyanomethanimine abundance ratio, where the gas-phase reaction between methanimine (CH2NH) and the cyanogen radical (CN) arises as the primary formation route.

Analysis on high-resolution spectrum of the S1-S0 transition of free-base phthalocyanine.

A high-resolution absorption spectrum of the S1-S0 transition of free-base phthalocyanine was observed and analyzed with improved reliability. The spectrum, with a partially resolved rotational structure, was obtained by using the buffer-gas cooling technique and a single-mode tunable laser. Our new analysis reveals that the S1←S0000 band belongs to the a-type transition, where the electronic transition moment aligns parallel to the NH-HN direction, allowing the assignment of the S1 state to 1B3u. These results agree with a prior study using supersonic expansion and are well supported by theoretical calculations. Interestingly, the rotational constant B in the S1 state, which is often smaller than that in the ground state for typical molecules, was found to be slightly larger than that in the S01Ag state. This suggests a change in the character of π bonds with the electronic excitation.

Fourier-transform microwave spectroscopy of the ClSO radical.

Pure rotational transitions of the ClSO radical have been observed by Fourier-transform microwave spectroscopy. a-type and b-type transitions, for both 35Cl and 37Cl isotopologues, were detected, and the observed very complicated fine and hyperfine components were assigned well. The intensities of the observed spectra of the two isotopologues correspond to the ratio of the isotope abundances of 35Cl and 37Cl. A total of 21 molecular constants were determined precisely for both 35ClSO and 37ClSO, including the rotational constants, centrifugal distortion constants, electronic spin-rotation constants, nuclear spin-rotation constants, magnetic hyperfine constants, and quadrupole coupling constants of chlorine. The molecular constants show ClSO to have the 2A'' electronic ground state with an out-of-plane unpaired electron. The spin density of the chlorine atom is about 10.6%, which is similar to that of the fluorine atom for FSO, about 8%. Results of the ClSO radical are compared with those of other triatomic radicals with similar structures, the XSS, XSO, and XOO radicals with X = H, F, and Cl, leading to a conclusion that the ClSO radical is more like FSO, but fairly different from the FOO and ClOO radicals.

Reinvestigation of the Fourier transform microwave spectrum of N-ethylacetamide: Pseudo a-type transitions

We observed ten pseudo a-type transitions in the E state of N-ethylacetamide (NEAA) using a Fourier transform microwave spectrometer. We found triplets caused by the coupling between the two methyl groups for the observed pseudo a-type transitions. Also, we succeeded in observing similar triplets for the 5 a-type Q-branch transitions, 20 b-type and 7 c-type transitions of the E state of NEAA. These observed triplets allowed us to determinate the potential barrier V3 to internal rotation of the ethyl-methyl group to be 1078 (12) cm−1. In addition, though the observed intensities of the c-type transitions in the A state were weak, the c-type transition in the E state were observed to be as intense as the b-type transitions of the E state. We explained in this paper the abnormal intensities of the c-type transitions and pseudo a-type transitions in the E state of NEAA.

Quantum monodromy in NCNCS - direct experimental confirmation.

Since the first confirmation of quantum monodromy in NCNCS (B. P. Winnewisser et al., Report No. TH07 in 60th International Symposium on Molecular Spectroscopy, Columbus, OH, (2005) and B. P. Winnewisser et al., Phys. Rev. Lett., 2005, 95, 243002) we have continued to explore its implications for the quantum structure of molecules. To confirm quantum monodromy bending-vibrational + axial-rotational quantum energy level information is needed. This was not directly available from the pure a-type rotational transitions available in 2005. The confirmation of quantum monodromy therefore had to be based on the fitting of the Generalised SemiRigid Bender (GSRB) model to the experimental rotational data. The GSRB is a physically motivated model and was able to extract the required information based on the changes of the rotational energy level structure upon excitation of the bending vibration and of the axial rotation. These results were, in some sense, predictions. Our goal here was to obtain a fully experimental and unambigous confirmation of quantum monodromy in NCNCS. This involved a series of experimental campaigns at the Canadian Light Source (CLS) synchrotron. To coax the required information out of the masses of spectral data that had been obtained a variety of techniques had to be used. The result is that we can now confirm, without recourse to a theoretical model, the existence of quantum monodromy in the ν7 bending mode of NCNCS. As a side benefit we also confirm the power of the GSRB model to extract the required information from the previously available data. The predictions previously provided by the GSRB were surprisingly accurate. Only a slight augmentation of the model was required to allow us to refit it including the new data, while maintaining the quality of the fitting for that data previously available. We also present a very basic introduction to the idea of monodromy and to how the GSRB was used.

Electronic, vibrational, and rotational analysis of 1,2-benzanthracene by high-resolution spectroscopy referenced to an optical frequency comb.

The electronic and vibrational structures of 1,2-benzanthracene-h12 (aBA-h12) and 1,2-benzanthracene-d12 (aBA-d12) were elucidated by analyzing fluorescence excitation spectra and dispersed fluorescence spectra in a supersonic jet on the basis of DFT calculation. We also observed the high-resolution and high-precision fluorescence excitation spectrum of the S1←S000 0 band, and determined the accurate rotational constants in the zero-vibrational levels of the S0 and S1 states. In this high-resolution measurement, we used a single-mode UV laser whose frequencies were controlled with reference to an optical frequency comb. The inertial defect is negligibly small, the molecule is considered to be planar, and the obtained rotational constants were well reproduced by the equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculation. Both a-type and b-type transitions are found to be included in the rotationally resolved spectrum, and the a-type contribution is dominant, that is, the transition moment is nearly parallel to the long axis of the aBA molecule. We concluded that the S1 state is mainly composed of the Φ(B) configuration. The observed fluorescence lifetime (106ns) is considerably longer than that of the Φ(A) system, such as anthracene (18ns). The transition moment for the lower state of mixed states becomes small, reflecting a near-cancelation of the contributions from the parts of the wavefunction corresponding to the two electronic configurations. The bandwidth of the S2 ← S0 transition is large, and the structure is complicated. It is attributed to vibronic coupling with the high vibrational levels of the S1 state.

Broadband microwave spectroscopy of cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane

The broadband microwave spectra of two related organosilicon molecules, cyclopentylsilane and 1,1,1-trifluorocyclopentylsilane, have been measured and assigned. Both molecules exhibit a Cs-symmetric “envelope” structure, which is confirmed by a determination of the heavy atom backbone rs substitution structures using isotopic substitution in natural abundance, as well as the presence of strong a-type transitions, weaker c-type, and a non-detection of b-type transitions, indicative of a mirror plane perpendicular to the prolate symmetry axis of the molecule. Both molecules appear to have significant zero-point vibrational averaging of their ring twisting and puckering coordinates, which lead to deviations between the ground state experimental structure and the predicted equilibrium structures at the B3LYP-D3/def2-TZVP level of theory. Additionally, we observe an excited vibrational state associated with this large amplitude motion and we use a simple perturbation theory argument to confirm the identity of these excited state carriers.

Magnetic dilution effect and topological phase transitions in (Mn1−xPbx)Bi2Te4

As the first intrinsic antiferromagnetic (AFM) topological insulator (TI), MnBi$_2$Te$_4$ has provided a material platform to realize various emergent phenomena arising from the interplay of magnetism and band topology. Here by investigating (Mn$_{1-x}$Pb$_x$)Bi$_2$Te$_4$ $(0\leq x \leq 0.82)$ single crystals via the x-ray, electrical transport, magnetometry and neutron measurements, chemical analysis, external pressure, and first-principles calculations, we reveal the magnetic dilution effect on the magnetism and band topology in MnBi$_2$Te$_4$. With increasing $x$, both lattice parameters $a$ and $c$ expand linearly by around 2\%. All samples undergo the paramagnetic to A-type antiferromagnetic transition with the N$\acute{e}$el temperature decreasing lineally from 24 K at $x=0$ to 2 K at $x=0.82$. Our neutron data refinement of the $x=0.37$ sample indicates that the ordered moment is 4.3(1)$\mu_B$/Mn at 4.85 K and the amount of the Mn$_{\rm{Bi}}$ antisites is negligible within the error bars. Isothermal magnetization data reveal a slight decrease of the interlayer plane-plane antiferromagnetic exchange interaction and a monotonic decrease of the magnetic anisotropy, due to diluting magnetic ions and enlarging the unit cell. For $x=0.37$, the application of external pressures enhances the interlayer antiferromagnetic coupling, boosting the N$\acute{e}$el temperature at a rate of 1.4 K/GPa and the saturation field at a rate of 1.8 T/GPa. Furthermore, our first-principles calculations reveal that the band inversion in the two end materials, MnBi$_2$Te$_4$ and PbBi$_2$Te$_4$, occurs at the $\Gamma$ and $Z$ point, respectively, while two gapless points appear at $x = $ 0.44 and $x = $ 0.66, suggesting possible topological phase transitions with doping.

Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin.

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G), syringol (2,6-dimethoxy phenol, S), 4-methyl guaiacol (MG), and 4-vinyl guaiacol (VG), under jet-cooled conditions in the gas phase. Using a combination of 13 C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the rC(1)-C(6) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at rC(1)-C(2) , rC(3)-C(4) and rC(5)-C(6) . Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm-1 , a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH⋅⋅⋅OCH3 H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V3 =67 cm-1 . Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.

Fine and hyperfine coupling constants of the cis-β-cyanovinyl radical, HCCHCN.

A Fourier-transform microwave spectrum of the cis-β-cyanovinyl radical is re-measured for the Ka = 0 ladder of the a-type transitions up to 30 GHz and the 212-111 transition at 19.85 GHz. Four b-type transitions are also observed using a MW-MW double-resonance technique. Fine and hyperfine components observed for each rotational transition are fully assigned in the present study, and the precise molecular constants are determined for the radical. From the comparisons of the hyperfine coupling constants with those of the vinyl radicals, it is concluded that the substitution of one of the β-hydrogens by the cyano group has little effect on the electronic structure of the vinyl radical.

Pure rotational spectrum and structural determination of 1,1-difluoro-1-silacyclopentane

The ground state, pure rotational spectrum of 1,1-difluoro-1-silacyclopentane has been studied using chirped-pulse, Fourier transform microwave (CP-FTMW) spectroscopy and observed in the 6-20.3 GHz region of the electromagnetic spectrum. This spectrum was acquired leveraging the deep averaging capability of the technique. The parent species, 13C, 29Si, and 30Si singly substituted isotopologues were observed in natural abundance and are reported. Only one conformer, the C2 conformer (half-chair), was observed. This is confirmed with a determined CCCC dihedral angle of -48.1(11)°. The spectrum is comprised of entirely a-type transitions in accordance with quantum chemical calculations. Multiple split transitions are present in the spectrum which have been attributed to a ring-twisting of the carbon atoms attached to the silicon atom in the ring. This motion has the carbons crossing the a-axis in the bc-plane leading to an inversion potential. Potential energy surfaces for the ring-twisting motion were undertaken and the experimentally determined energy level difference observed in comparison to these surfaces is reasonable. A Kraitchman analysis of the experimentally determined, singly substituted isotopologues is in agreement with the optimized, twisted (nonplanar) equilibrium structure. This structure has been compared to other similar silicon-containing ring molecules using second moment arguments and these comparisons are discussed.

Measurements of microwave and NMR spectra for 15N substituted formamidinium formate

A sample of doubly-substituted 15N formamidinium formate was synthesized and the microwave spectrum was measured in the 5.6–14.2 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (MW) spectrometer. A total of 13 a-type rotational transitions were measured and fitted to obtain the rotational constants and centrifugal distortion constants for the15N substituted isotopologue. The rotational constants and centrifugal distortion constants determined have the following values: A = 5808.02(18) MHz, B = 2127.008(2) MHz, C = 1557.615(2) MHz, DJ = 0.60(11) kHz, DJK = 4.95(71) kHz and DK = −0.138(24) kHz. Tunneling splittings were searched for, but not observed. NMR spectra for this compound are reported including values of δC, δN, 1JCH, 1JCN and 2JNH in CD3SOCD3, CD3OD and D2O. The occurrence of NH to ND exchange in D2O allowed the fortuitous measurement of 1JND.

Fourier transform infrared (FTIR) spectroscopy of formaldoxime (CH2NOH) in the 450–3800 cm−1 region and its [formula omitted] band

The Fourier transform infrared (FTIR) spectrum of formaldoxime (CH2NOH) was recorded in the 700–3800 cm−1 region at the Australian Synchrotron with an unapodized resolution of 0.5 cm−1 to identify its fundamental and overtone bands and to measure their relative infrared (IR) intensities in this region. The high-resolution FTIR spectrum of the ν9 band of CH2NOH was also recorded with an unapodized resolution of 0.00096 cm−1 in the 450–600 cm−1 region for a rovibrational analysis. In the analysis of the A/B-hybrid type ν9 band, a total of 2011 infrared transitions (1655b-type and 356 a-type transitions) were fitted using the Watson's A-reduced and S-reduced Hamiltonians in the Ir representation with a root-mean-square (rms) deviation of 0.000096 cm−1 and 0.000305 cm−1 respectively. From this rovibrational analysis, the v9 = 1 state rotational constants (A, B and C) and five quartic terms (ΔJ, ΔJK, ΔK, δJ, δK) were improved from previous work and three sextic terms (ΦJK, ΦKJ, ΦK) were derived for the first time. The band center of the ν9 band of CH2NOH was found to be 530.0187764(63) cm−1 and 530.018762(18) cm−1 in the A-reduced and S-reduced Hamiltonians respectively. All five quartic and three sextic (ΦJK, ΦKJ, ΦK) centrifugal distortion terms of the ground state of CH2NOH were also improved with higher accuracy. They were obtained through the fitting of 3020 ground state combination differences (GSCDs) derived from the IR transitions of the ν9, ν12 and ν10 bands of CH2NOH, together with 23 previously reported microwave frequencies. The rms deviation of this GSCD fit was 0.000498 cm−1 and 0.000519 cm−1 in the A-reduced and S-reduced Hamiltonians respectively. Additionally, rotational constants and higher order centrifugal distortion terms of the ground and v9 = 1 states were computed from theoretical anharmonic calculations at two different levels of theory, B3LYP and MP2 with the cc-pVTZ basis set, for comparison with the experimental results. The calculated and experimental rovibrational constants of CH2NOH for both ground and v9 = 1 states were in good agreement.

Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes

High level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF2) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl2). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr2) was obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm-1 barrier to planarity, 2SiCPF2 has a planar ring system, 2SiCPCl2 has a calculated tiny 4 cm-1 barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm-1. Microwave spectra of seven isotopic species of 2SiCPF2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The a-type and b-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, 29Si, 30Si, and all singly-substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.

Ro-vibrational spectroscopy of 2,5-norbornadiene between 400 and 850 cm−1

Rotationally-resolved infrared absorption spectra of the two lowest-energy fundamental bands for each of the three infrared active symmetry species of 2,5-norbornadiene (C2v point group symmetry; asymmetry parameter κ = −0.2203), ν11 (a1), ν12(a1), ν28(b1), ν29(b1), ν38(b2), and ν39(b2), have been measured, assigned and fit to a semi-rigid rotor Hamiltonian to produce a set of spectroscopic constants for each band. The fit band centers are ν11 (a1), c-type transitions, 728.852357(3) cm−1; ν12 (a1), c-type transitions, 417.31604(1) cm−1; ν28 (b1), a-type transitions, 656.719741(3) cm−1; ν29 (b1), a-type transitions, 500.31425(1) cm−1; ν38 (b2), b-type transitions, 800.82243(2) cm−1; and ν39 (b2), b-type transitions, 539.23110(6) cm−1. Using ground state combination differences from the ν11 and ν28 bands and previously published microwave data, an improved set of ground state spectroscopic constants was fit for the parent isotopologue of the molecule.

Strain control of phase transition and magnetocaloric effect in Nd0.5Sr0.5MnO3 thin films

Phase transition and the magnetocaloric effect (MCE) in Nd0.5Sr0.5MnO3 (NSMO) epitaxial thin films were tailored through controlling the lattice-mismatch-induced-strain by depositing on (011)—(La0.18Sr0.82)(Al0.59Ta0.41)O3 and SrTiO3 (STO) single crystalline substrates, respectively. The NSMO film grown on STO, exhibiting uniaxial like tensile strain of 1.3% along the in-plane [100] direction, undergoes a paramagnetic to ferromagnetic transition at ∼210 K followed by a ferromagnetic to A-type antiferromagnetic transition at ∼179 K upon cooling; meanwhile, the film grown on LSAT, exhibiting anisotropic in-plane tensile strains of 0.36% along [100] and 0.50% along [01¯1] directions, undergoes further transition to CE-type antiferromagnetic transition at ∼145 K. NSMO/LSAT with such transitions facilitates a strong MCE over a much wider temperature range from 90 to 170 K, with the magnetic entropy change comparable to the recently reported La0.25Ca0.75MnO3 bulk. These findings suggest that control of strain in manganite films with first-order phase transition is a feasible way to broaden their MCE temperature range.

Millimeter-wave spectroscopy of HDC=CH.

Rotational transitions of the mono(β)-deuterated vinyl radical, HDC=CH, produced in a supersonic jet expansion by the ArF excimer laser photolysis, were observed by millimeter-wave spectroscopy. The b-type rotational transitions together with the weak a-type transitions were observed only for the lower component of the tunneling doublet, and no tunneling-rotation transitions connecting the lower and upper components were observed, suggesting that state mixing between the two components is negligibly small. The derived molecular constants such as the A rotational constant, Fermi contact interaction constants, and magnetic dipolar interaction constants indicate that the carrier of the observed spectrum is the trans-form of HDC=CH isomers, where the α-proton is located on the opposite side of the β-deuteron. The present conclusion of the trans-form of HDC=CH was also supported by the ab initio calculation in the CCSD(T)/cc-pVTZ level since the trans-form is calculated to be located by 30.04 cm-1 lower than the cis-form due to the difference in the zero point energy. As a result, the tunneling components in the ground state of HDC=CH behave as two different isomers localized at the trans- and cis-wells of the asymmetric double minimum potential. Observed hyperfine constants for HDC=CH were compared with those for H2C=CH to be consistent with each other.

Vinyl cyanide (CH2CHCN) in interstellar space: potential spectral lines for its detection

Vinyl cyanide is a molecule having planar geometry with electric dipole moment components, μa=3.821D and μb=0.687D. Thus, a-type transitions are very strong as compared to b-type transitions, and are considered in this investigation. The rotational levels for a-type transitions, may be divided into two distinct groups: one group having odd values of ka and the other having even values of ka. Using the known values of rotational and centrifugal distortion constant along with the electric dipole moment μa, we have calculated energies of lower 120 rotational levels, having energy up to 92cm−1, for each group, and the probabilities for radiative transitions between the levels. These radiative transition probabilities in conjunction with the scaled values of collisional rate coefficients are used in the Sobolev Large Velocity Gradient analysis. Out of a large number of lines, we have selected the strongest ones. In the low lying levels, besides the 4 observed lines of CH2CHCN, we have discussed about 8 additional transitions, of which 2 showing the phenomenon of anomalous absorption and 6 showing emission feature are found. These lines may play important role for the search of vinyl cyanide in a cosmic object.

Transfer of radiation in the formic acid: A precursor for amino acids

Formic acid (HCOOH), a simplest carboxylic acid, has great importance as it is a precursor for amino acids (constituents of life). It has two rotameric isomers: trans-HCOOH and cis-HCOOH, each of which lies in a plane due to the delocalization of $$\pi $$ -electrons over the heavy atom chain. In each of the isomers, the electric dipole moment is aligned such that there are both a and b type rotational transitions. Further, the energy levels in each type of transitions can be classified into two groups. Thus, there are 8 groups in which the rotational transitions of formic acid may be classified. The trans-HCOOH is detected in Sgr B2, cold dark cloud L134N, Sgr A, comet Hale-Bopp, Orion KL, W51, IRAS 16293-2422 through its a-type transitions. Because of very small value of b-component of electric dipole moment, the b-type transitions of trans-HCOOH may not be detected. To our knowledge, no transitions of cis-HCOOH are yet detected in the interstellar medium, though a and b components of its electric dipole moment are quite large. Using spectroscopic data of trans-HCOOH and cis-HCOOH, we have calculated energies of 100 rotational levels for each of the 8 groups, and the radiative transition probabilities (Einstein A and B coefficients) for radiative transitions between the levels. Since the rate coefficients for collisional transitions between the levels are not available, by using the scaled values for them along with the radiative transition probabilities, we have solved a set of 100 statistical equilibrium equations coupled with the equations of radiative transfer for each group. We have investigated intensities of 16 observed a-type transitions and 12 b-type transitions of trans-HCOOH. We have also found six transitions, $$1_{1 0} - 1_{1 1}$$ (1.405 GHz), $$2_{1 2} - 3_{0 3}$$ (7.545 GHz), $$3_{1 2} - 3_{0 3}$$ (79.744 GHz), $$3_{2 1} - 3_{1 2}$$ (222.287 GHz), $$1_{1 1} - 2_{0 2}$$ (30.843 GHz) and $$4_{1 3} - 4_{0 4}$$ (82.740 GHz) of cis-HCOOH showing anomalous absorption and nine transitions $$4_{1 4} - 3_{1 3}$$ (85.042 GHz), $$5_{1 5} - 4_{1 4}$$ (106.266 GHz), $$3_{0 3} - 2_{0 2}$$ (65.840 GHz), $$4_{0 4} - 3_{0 3}$$ (87.694 GHz), $$5_{0 5} - 4_{0 4}$$ (109.470 GHz), $$5_{0 5} - 4_{1 4}$$ (40.778 GHz), $$7_{0 7} - 6_{1 6}$$ (90.910 GHz), $$4_{0 4} - 3_{1 3}$$ (16.350 GHz) and $$6_{0 6} - 5_{1 5}$$ (65.661 GHz) of cis-HCOOH showing emission feature. These transitions of cis-HCOOH in addition to those of trans-HCOOH may help in the identification of HCOOH in a cosmic object.