1 Introduction

The interstellar matter in the inner few hundreds parsecs of the Galaxy (hereafter GC) is mainly molecular. In this region there are molecular clouds and huge cloud complexes like Sgr B2 which can be as large as 70 pc, with masses of 10⁶ solar masses. The physical conditions in the GC clouds differ appreciably to those of the clouds of the disk of the Galaxy. The GC clouds have average densities of $\sim \,$10⁴ cm^-3 instead of 10² cm^-3 typical of the clouds of the disk. In addition, with widespread high temperatures (up to 200 K), GC clouds are hotter than disk clouds.

The temperatures of the warm gas are known mainly by observations of ammonia (NH₃) metastable lines. Güsten et al. (1981, 1985) derived rotational temperatures ( $T_{\rm rot}$) of 60-120 K in several GC clouds, most of them in the Sgr A complex. Morris et al. (1983) showed that $T_{\rm rot} \sim 30$-60 K are common in the region $\vert l\vert<2^\circ$. The most complete study of the temperature structure of the molecular gas in the GC, was carried out by Hüttemeister et al. (1993). They presented a multilevel study of NH₃ metastable lines of 36 molecular clouds distributed all along the "Central Molecular Zone" (CMZ, in notation of Morris & Serabyn 1996) and the "Clump 2" complex, which, although not belonging to the actual CMZ, exhibits similar properties. They detected warm gas at all galactic longitudes and showed that the NH₃ emission can be characterized by two temperature components since the $T_{\rm rot}$ derived from the (1,1) and (2,2) levels is $\sim \,$20-30 K and that derived from the (4,4) and (5,5) levels is $\sim \,$70-200 K. Unfortunately, the a priori unknown abundance of the NH₃ molecule has made it difficult to estimate the total column density of warm gas in the GC clouds.

The heating of the molecular gas over large regions ($\sim \,$10 pc) where the dust temperature is lower than 30 K (Odenwald & Fazio 1984; Cox & Laureijs 1989; Martín-Pintado et al. 1999a; Rodríguez-Fernández et al. 2000) is a puzzle. Indirect arguments such as the large widths of molecular lines or large abundances in gas phase of molecules such as SiO (Martín-Pintado et al. 1997; Hüttemeister et al. 1998) or NH₃ points towards a mechanical heating. Wilson et al. (1982) proposed the dissipation of turbulence induced by differential Galactic rotation as a possible heating source.

For the first time, we have measured the total column densities of warm gas in the GC clouds by observing the lowest pure-rotational transitions with the Infrared Space Observatory (ISO; Kessler et al. 1996). The pure-rotational lines trace gas with temperatures of a few hundred K (see Shull & Beckwith 1982 for a review on the properties and the notation of the molecule). ISO has detected pure-rotational lines in a variety of sources such as: Young Stellar Objects (van den Ancker 1999); galactic nuclei (see e.g. Kunze et al. 1999); Photo Dissociation Regions (PDRs) like NGC 7023 (Fuente et al. 1999, 2000) or S140 (Timmermann et al. 1996); shock-excited sources such as Orion Peak 1 (Rosenthal et al. 2000); and proposed X-ray excited regions (XDRs) like RCW 103 (see Wright 2000).

Our sample consists of 18 molecular clouds from the surveys of Hüttemeister et al. (1993) and Martín-Pintado et al. (1997). Two of these show a non-equilibrium ortho-to-para ratio and have been studied in detail by Rodríguez-Fernández et al. (2000). In this paper we present the other 16 clouds of the sample. The clouds are distributed along the CMZ, from the Sgr E region to the vicinity of Sgr D and the "Clump 2" complex. Four clouds are located in the Sgr C complex, three in the vicinity of Sgr A (two are in the radio Arc). Two clouds are situated in the cold dust ridge reported by Lis & Carlstrom (1994) that seems to connect the radio Arc and Sgr B. Other three clouds belong to the Sgr B complex.

This paper is organized as follows. In Sect. 2 we present and IRAM-30 m observations and ISO observations. The analysis of the CO isotopes and , is presented in Sects. 3 and 4, respectively. The results and the possible heating mechanism of the warm gas are discussed in Sect. 5.