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 $$\pi $$π-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, $$1_{1 0} - 1_{1 1}$$110-111 (1.405 GHz), $$2_{1 2} - 3_{0 3}$$212-303 (7.545 GHz), $$3_{1 2} - 3_{0 3}$$312-303 (79.744 GHz), $$3_{2 1} - 3_{1 2}$$321-312 (222.287 GHz), $$1_{1 1} - 2_{0 2}$$111-202 (30.843 GHz) and $$4_{1 3} - 4_{0 4}$$413-404 (82.740 GHz) of cis-HCOOH showing anomalous absorption and nine transitions $$4_{1 4} - 3_{1 3}$$414-313 (85.042 GHz), $$5_{1 5} - 4_{1 4}$$515-414 (106.266 GHz), $$3_{0 3} - 2_{0 2}$$303-202 (65.840 GHz), $$4_{0 4} - 3_{0 3}$$404-303 (87.694 GHz), $$5_{0 5} - 4_{0 4}$$505-404 (109.470 GHz), $$5_{0 5} - 4_{1 4}$$505-414 (40.778 GHz), $$7_{0 7} - 6_{1 6}$$707-616 (90.910 GHz), $$4_{0 4} - 3_{1 3}$$404-313 (16.350 GHz) and $$6_{0 6} - 5_{1 5}$$606-515 (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.

中文翻译：

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甲酸中的辐射转移：氨基酸的前体

甲酸 (HCOOH) 是一种最简单的羧酸，具有重要意义，因为它是氨基酸（生命成分）的前体。它有两个旋转异构体：反式 HCOOH 和顺式 HCOOH，由于 $$\pi $$π-电子在重原子链上的离域，每个异构体都位于一个平面内。在每个异构体中，电偶极矩排列成 a 和 b 型旋转跃迁。此外，每种跃迁的能级可以分为两组。因此，甲酸的旋转转变可分为 8 个组。在人马座 B2、冷暗云 L134N、人马座 A、彗星 Hale-Bopp、Orion KL、W51、IRAS 16293-2422 中通过其 a 型跃迁检测到反式 HCOOH。由于电偶极矩的 b 分量值非常小，可能无法检测到反式 HCOOH 的 b 型跃迁。据我们所知，尽管其电偶极矩的 a 和 b 分量相当大，但尚未在星际介质中检测到顺式 HCOOH 的跃迁。使用反式 HCOOH 和顺式 HCOOH 的光谱数据，我们计算了 8 个组中每一个的 100 个旋转能级的能量，以及能级之间辐射跃迁的辐射跃迁概率（爱因斯坦 A 和 B 系数）。由于水平之间碰撞跃迁的速率系数不可用，通过使用它们的缩放值以及辐射跃迁概率，我们已经解决了一组 100 个统计平衡方程，以及每组的辐射转移方程。我们研究了反式 HCOOH 的 16 个观察到的 a 型跃迁和 12 个 b 型跃迁的强度。我们还发现了六个跃迁，$$1_{1 0} - 1_{1 1}$110-111 (1.405 GHz)、$$2_{1 2} - 3_{0 3}$$212-303 (7.545 GHz) , $3_{1 2} - 3_{0 3}$312-303 (79.744 GHz), $3_{2 1} - 3_{1 2}$321-312 (222.287 GHz), $1_{1 1} - 2_{0 2}$111-202 (30.843 GHz) 和 $4_{1 3} - 4_{0 4}$413-404 (82.740 GHz) 顺式 HCOOH 显示异常吸收和九个跃迁 $$4 _{1 4} - 3_{1 3}$414-313 (85.042 GHz)，$5_{1 5} - 4_{1 4}$515-414 (106.266 GHz)，$3_{0 3} - 2_{0 2}$303-202 (65.840 GHz)，$4_{0 4} - 3_{0 3}$404-303 (87.694 GHz)，$5_{0 5} - 4_{0 4}$ 505-404 美元（109.470 GHz）、5_{0 5} - 4_{1 4}505-414 美元（40.778 GHz）、7_{0 7} - 6_{1 6}707-616 美元（90.910 GHz） )、$4_{0 4} - 3_{1 3}$404-313 (16.350 GHz) 和 $6_{0 6} - 5_{1 5}$606-515 (65. 661 GHz) 的 cis-HCOOH 显示发射特征。除了反式 HCOOH 之外，顺式 HCOOH 的这些跃迁可能有助于识别宇宙物体中的 HCOOH。