We have observed infrared absorption lines of C2H2, HCN, OCS, CO, and NH3 toward the infrared cluster in the Orion molecular cloud. We have detected many lines at 13.5-mu-m in the nu-5 vibrational band of C2H2 toward IRc2 and IRc7. Based on the relative line strengths and our detection of several (CCH2)-C-13 lines, we believe that the lines are narrow (Gaussian dispersion, sigma-upsilon approximately 1 km s-1) and saturated. The derived temperature is about 140-150 K, and the column density of C2H2 is 1.4 x 10(17) cm-2 toward IRc2, with about half as much C2H2 at a similar temperature toward IRc7. We have also detected lines at 7.6-mu-m of the nu-4 + nu-5 vibrational band of C2H2 toward IRc2; the temperature of the C2H2 derived from this band is consistent with that derived from the nu-5 band, but the column density is about 3 times larger. We have also observed several lines at 13.5-mu-m of the nu-2 vibrational band of HCN toward IRc2 and IRc7. The observed line strengths toward both sources indicate that HCN is in LTE for rotational levels through J = 14. The temperature is about 150 K, consistent with the temperature derived from C2H2, and the column density of HCN toward IRc2 is about 2.6 x 10(17) cm-2. Toward IRc2, absorption from the J = 17 level is also detected, but the level is subthermally excited, consistent with a gas density between 3 x 10(6) cm-3 and 1 x 10(7) cm-3. We have also detected many lines at 4.9-mu-m of the nu-1 band of OCS toward IRc2. The derived temperature is 140 K, consistent with the temperatures derived from C2H2 and HCN. The derived column density of OCS is 3.5 x 10(16) cm-2. A few lines of CO, (CO)-C-13, and (probably) (CO)-O-18 have been detected toward IRc2; the resulting column density of CO (approximately 1.5 x 10(20) cm-2) is very uncertain because of confusion with other lines and P Cygni profiles. Three lines of NH3 at 11-mu-m, arising from the (J, K) = (2, 0), (2, 1), and (3, 3) levels, have also been detected toward both IRc2 and IRc7. The absorption most likely arises from gas in the plateau kinematic feature, but the hot core cannot be ruled out, except for OCS. The column densities of HCN and OCS are considerably greater than the typical estimates from millimeter or submillimeter emission lines, as expected if the matter is concentrated around the infrared source. The abundances of all these species are quite high. In particular, no steady state gas-phase model seems capable of explaining the C2H2 abundance relative to H-2 (approximately 10(-6)), but models in which the early, high abundances of C2H2 are frozen onto dust grains and released later when the dust is warmed do predict this enhanced abundance.
