Isopropyl cyanide was recently detected in space as the first branched alkyl compound. Its abundance with respect to n-propyl cyanide in the Galactic center source Sagittarius B2(N2) is about 0.4. Astrochemical model calculations suggest that for the heavier homologue butyl cyanide the branched isomers dominate over the unbranched n-butyl cyanide and that 2-cyanobutane is the most abundant isomer. We have studied the rotational spectrum of 2-cyanobutane between 2 and 24 GHz using Fourier transform microwave spectroscopy and between 36 and 402 GHz employing (sub)millimeter absorption spectroscopy. Transitions of the lowest energy conformer were identified easily. Its rotational spectrum is very rich, and the quantum numbers J and K_a reach values of 111 and 73, respectively. This wealth of data yielded rotational and centrifugal distortion parameters up to tenth order, diagonal and one off-diagonal ¹⁴N nuclear quadrupole coupling parameters, and one nuclear spin–rotation coupling parameter. We have also carried out quantum chemical calculations in part to facilitate the assignments. The molecule 2-cyanobutane was not found in the present ALMA data of Sagittarius B2(N2), but it may be found in the more sensitive data that have been completed very recently in the ALMA Cycle 4.

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2-氰基丁烷的最低能构型的旋转光谱

最近在太空中检测到异丙基氰化物是第一个支链烷基化合物。在银河系中心射手座B2（N2）中，其相对于正丙基氰的丰度约为0.4。天体化学模型计算表明，对于较重的同系物丁基氰化物，支链异构体比非支链正丁基氰化物支配，而2-氰基丁烷是最丰富的异构体。我们已经使用傅里叶变换微波光谱学研究了2和24 GHz之间的2-氰基丁烷的旋转光谱，并使用（亚）毫米波吸收光谱学研究了36和402 GHz之间的2-氰基丁烷的旋转光谱。最低能量构象异构体的转变很容易识别。它的旋转光谱非常丰富，并且量子数J和K _a分别达到111和73的值。大量的数据产生了高达十阶的旋转和离心畸变参数，对角线和一个非对角线的¹⁴ N核四极耦合参数，以及一个核自旋-旋转耦合参数。我们还进行了部分量子化学计算，以简化分配过程。在目前的人马座​​B2（N2）的ALMA数据中未发现分子2-氰基丁烷，但在最近的ALMA Cycle 4中完成的更敏感的数据中却发现了该分子。