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We have mapped 27 massive star-forming regions associated with water masersusing three dense gas tracers: HC3N 10-9, HNC 1-0 and C2H 1-0. The FWHM sizesof HNC clumps and C2H clumps are about 1.5 and 1.6 times higher than those ofHC3N, respectively, which can be explained by the fact that HC3N traces moredense gas than HNC and C2H. We found evidence for increase in optical depth ofC2H with `radius' from center to outer regions in some targets, supporting thechemical model of C2H. The C2H optical depth is found to decline as molecularclouds evolve to later stage, suggesting that C2H might be used as "chemicalclock" for molecular clouds. Large-scale kinematic structure of clouds wasinvestigated with three molecular lines. All these sources show significantvelocity gradients. The magnitudes of gradient are found to increase towardsthe inner region, indicating differential rotation of clouds. Both the ratio ofrotational to gravitational energy and specific angular momentum seem todecrease toward the inner region, implying obvious angular momentum transfer,which might be caused by magnetic braking. The average magnetic field strengthand number density of molecular clouds is derived using the uniformly magneticsphere model. The derived magnetic field strengths range from 3 to 88 \mu G,with a median value of 13 \mu G. The mass-to-flux ratio of molecular cloud iscalculated to be much higher than critical value with derived parameters, whichagrees well with numerical simulations.

Large-Scale Kinematics, Astrochemistry and Magnetic Field Studies of Massive Star-forming Regions through HC3N, HNC and C2H Mappings