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We have performed a high resolution 4-13 ${\mu}m$ spectral survey of the hotmolecular gas associated with the massive protostars AFGL 2591 and AFGL 2136,utilising the Echelon-Cross-Echelle-Spectrograph (EXES) on-board theStratospheric Observatory for Infrared Astronomy (SOFIA), and the iSHELLinstrument and Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASAInfrared Telescope Facility (IRTF). Here we present results of this survey withanalysis of CO, HCN, C$_2$H$_2$, NH$_3$ and CS, deriving the physicalconditions for each species. Also from the IRTF, iSHELL data at 3 ${\mu}m$ forAFGL 2591 are presented that show HCN and C$_2$H$_2$ in emission. In the EXESand TEXES data, all species are detected in absorption, and temperatures andabundances are found to be high (600 K and 10$^{-6}$, respectively).Differences of up to an order of magnitude in the abundances of transitionsthat trace the same ground state level are measured for HCN and C$_2$H$_2$. Themid-infrared continuum is known to originate in a disk, hence we attribute theinfrared absorption to arise in the photosphere of the disk. As absorptionlines require an outwardly decreasing temperature gradient, we conclude thatthe disk is heated in the mid-plane by viscous heating due to accretion. Weattribute the near-IR emission lines to scattering by molecules in the upperlayers of the disk photosphere. The absorption lines trace the disk propertiesat 50 AU where a high temperature gas-phase chemistry is taking place.Abundances are consistent with chemical models of the inner disk of Herbigdisks.

High Resolution Infrared Spectroscopy of Hot Molecular Gas in AFGL 2591  and AFGL 2136: Accretion in the Inner Regions of Disks Around Massive Young  Stellar Objects