1. Introduction

2. Observations
2.1. Technical Details
2.2. Source Selection

3. Results
3.1. Ortho ${\bf\cdot}{\mathrm{H}}_{2}{\bfD}^{+}$
3.2. Para ${\bf\nabla}\!\cdot\!{\bf D}_{2}{\mathrm{H}}^{+}$

4. Analysis
4.1. Derivation of the Average Ortho ${\bf\cdot}{\mathrm{H}}_{2}{\bfD}^{+}$ Column Densities
4.2. Evaluation of the Errors
4.3. Correlations

5. Chemical Discussion
5.1. Simple Theory: What Affects Deuterium Fractionation
5.2. Simple Chemical–Physical Model

6. Conclusions